An electronic color code is a system which wordlessly identifies either the functions of cables and connections, or the types and variants of electronic components. Codes for cables and connections range from the use of pink, blue, and green connectors on computer sound cards, to early attempts at standardizing the colors of battery, loudspeaker, and antenna wires of radio sets in the 1920s, to the 25-pair color code, used in telecommunications cables. Among the component identification applications, those used on resistors are most widespread, with usage on inductors also popular, while in the past, many variations were applied to capacitors, diodes, and other part types.
History
Before standardization, some manufacturers used their own color codings on the fixed resistors in their radio sets, seemingly choosing a random color for each part stock number. Then, in 1930, a new code started coming into wide usage among radio makers. This new style encoded the resistance value of each part by means of one background paint color and usually, two additional paint marks. The Radio Manufacturers Association (RMA) gave its endorsement of this coding around 1931. This all-over painted, "body-end-dot" coding style, while most vividly associated with pre-war radio equipment, did live on for a while, and was still in use in the 1960s.
In 1935, advertisements appeared for resistors coded with a sequence of color bands including one acting as a tolerance indicator; the RMA adopted a standard for this alternative style by 1941. A 1943 reference book, Mechanical Practice gives an early description of a further development. While the 1935 banded style only called for two digit bands before the band that specifies a "multiplying value", coded as an exponent, the new variation had three, bringing the total number of bands including the tolerance to five, as opposed to just three or four. This might seem to indicate that even at that early date, the use of a fourfive rule became necessary in decoding banded resistors; in going from four to five in the total count of bands, that is where the number of digit-encoding bands increases from two to three, moving the exponent band from the third to the fourth position. In practice this was not a very important rule to memorize back then, as actual resistors showing the three-digit coding innovation remained scarce for twenty years afterwards. Eight decades later, application of the rule in some form is routinely required, but now a big part of the art of reading random-sourced resistors is, recognizing the exceptions, those tricky cases where resistance codings do not follow the rule.
Over many decades, as the organization name changed (RMA, RTMA, RETMA, EIA) so was the name of the code. Though known most recently as EIA color code, the four name variations are found in books, magazines, catalogs, and other documents over more than years.
In 1952, the International Electrotechnical Commission (IEC) published a fork of the RMA's banded resistor concept as IEC 62:1952; although it did not expand to include three-digit codings following a fourfive rule until the 1974 edition, its successors and national versions/translations are widely referenced today. Starting with the 1968 edition it added in the widely used "RKM codes" for printed text component values on resistors and capacitors. Successive revisions, up to the current IEC 60062:2016, added more optional features to the resistor color code, but also diverged from the EIA versions.
One standardized color code scheme for capacitors was in use by 1934, although some radio makers kept using random-picked color codes on these longer than they did on resistors. (At that point, most were still calling these "condensers", an older term.) There were many variations of flat postage stamp/domino-style mica and paper dielectric capacitors with paint dots; tubular capacitors with a sequence of spots or bands also date to the 1940s. Eventually many more styles were produced using mica, paper, ceramic, glass, and plastics for the dielectric, also many tantalum types, not all codings directly published by the RMA and successors, some with dots, some with bands, some with layer-dipped paint. A 1949 article detailed nine marking types in use by that time, but the proliferation of styles was just getting started then. Some later ceramic-dielectric ones from the VCR era of roughly 1980 onward look a lot like modern small axial-leaded resistors. Today, while color-coded "caps" are less commonly encountered, the layer-dipped "tropical fish" ones are still preferred by some, and recent reproductions of certain old tubular capacitor types have been embellished with replica "bumble-bee" color bands, suitable to make any classic-style electric guitar more authentic.
Schemes for color-coding small inductors, (A.K.A. radio frequency chokes, coils, etc.) were in use by 1950; as with the capacitors, some types do look a lot like modern small resistors. For reading, while many recent ones decode like common resistors, giving an inductance value in microhenries, older ones and some current ones are more complicated; unlike resistors, a gold or silver band may be at either end, or both, and there may be no exponent band.
Small semiconductor diodes/rectifiers were being marked with color bands by 1958. Some Zener/avalanche-breakdown diodes have paint bands that indicate their main characteristic of interest, which is, their nominal rated Zener voltage. More commonly, a type number is indicated using one of several variants that support the U.S. JEDEC "1N" series (Glass diodes marked this way were common in the 1960s.) and European Pro Electron numbering schemes. Proprietary arbitrary codings were used by some; Hitachi made diodes with bands of designer colors such as "verdure" and "yellow ocher". Many later-production glass diodes have only one or two color bands, indicating a broad type category.
Several complex color coding systems for plastic-cased transistors were in use in the Soviet sphere. The U.S. standard for diodes allows application to transistors but seems to have been little used for this. Usually when color markings are found on U.S.-style small plastic transistors that have printed numbering also, they indicate a bin-sort code or something proprietary.
By 1970, disc-shaped thermistors and voltage dependent resistors/varistors were being made with color coding in the form of layer-dipped paint; these may be confused with some ceramic disk capacitors which are painted similarly. There are also tubular, axial-leaded styles of varistors which have their own look which differentiates them from other components.
There is even a color band coding standard for fuses, published in 1988 as IEC 127, and adopted in national variants covering both cartridge types that look like fuses and also, miniature types that look like modern small resistors.
The paint pigment used for the number seven, according to some British sources, is supposed to be a "dark violet"; others in the British Commonwealth have called it mauve. The general consensus of references world-wide through the decades would be a simple violet, although, in the early years there are several sources that call it purple, and even a vote for lavender may be found.
The color band system is easily readable by people with good color vision and from multiple directions, if well-chosen paints are used. Fairly early on, some manufacturers put printed numbers on as an additional identifying aid. Some might say the system is poorly designed, as it would be simple enough to choose ten colors that work for most color blind people.
Color bands were used because they were easily and cheaply printed on tiny components. However, there were drawbacks, especially for color blind people. Overheating of a component or dirt accumulation may make it impossible to distinguish brown from red or orange. Advances in printing technology have now made printed numbers more practical on small components. The values of components in many surface mount packages are marked with printed alphanumeric codes instead of a color code.
Resistors
The resistance value is given by two or three color markings (usually rings/bands starting at the head end, or rarer, partial bands or spots, or, in the oldest style, the body/background and optional end mark) indicating numeric digits plus one more giving a "multiplying value" which is specified by an exponent. The multiplying value is ten raised to the power of the exponent; for the ten positive valued colors that is just a one followed by zeroes. After the exponent, a tolerance indicator band and sometimes one more may follow, making up to six bands in total. The system is similar to scientific notation, but it differs in its normalization rules. In the normalized scientific notation representation of any number, the first nonzero digit is the "units," or "ones" place. A denormalized case, where some other position is the first nonzero digit, may often be encountered in the intermediate steps required when adding or multiplying two numbers, before the final step of re-normalizing. But in resistor coding, the first nonzero digit is the tens or hundreds place, depending on whether there are two or three digit bands, with exceptions of some low-end resistance values. These exceptions, where the sequence starts with black/zero, are the denormalized resistor codings, used when the lowest available color for the exponent is not low enough otherwise. For example, the number fifty-six thousand is in normalized scientific notation. A 2% tolerance resistor of that many ohms, not nearly low enough to need a black first band, may be coded with two or three digit bands; it may read:
with markings green(5)-blue(6)-orange(3)-red(2%) or,
with markings green(5)-blue(6)-black(0)-red(2)-red(2%).
This gives the two equivalent parts different colors for their exponent bands. Doing a rough sort of mixed resistors might involve using a row of jars, with one jar for each exponent color (third band) of the two-digit coded resistors, and also a second row with a jar for each exponent color (fourth band) of the three-digit coded ones. A 56000 ohm resistor might be then found in the 2 digit oranges or the 3 digit reds. The main difficulties in this process are getting the correct reading orientation on some of the smaller resistors, and, identifying each resistor as a two- or three-digit coding, to determine which row it goes in. If all the resistors were made conforming to the EIA and IEC standards, they would all follow the fourfive rule, where four and fewer bands always imply there are two digit bands before the exponent, while five or more always mean there are three digit bands. In practice, many resistors will be found where the rule has been flouted or disregarded.
Many call the numeric digits given by the up-front color bands "significant digits", "significant figures", or, in the current IEC standard, "significant numerals". Pedants might argue that the word "significant" is misleading unless much arcane explanatory text is added, (this being almost never provided,) so it might be better to use a term like "numeric digits," "marked digits," or "coded digits" instead.
Some sources give graphic decoding keys that make it look like, in going from a four- to a five-band resistor, the new band is added as the middle position, and going from a five- to a four-band resistor, it is the middle one that is lost. These may foster the false presumption that a five-band resistor whose middle band is black, which decodes to zero, should have the same value as the four-band code obtained by rubbing out the black band.