the resistor colour code

The colour code is the ring of coloured bands printed around a small resistor that tells you its value, because the parts are too tiny to print numbers on. Each colour stands for a digit, and you read the bands in order like reading a number off a ruler. It looks cryptic at first, but it is just a compact way to write a value and a tolerance on a 3 mm body.

On a common 4-band resistor, the first two bands are digits, the third is a multiplier (how many zeros, or power of ten), and the fourth is the tolerance. The digit colours run: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. So brown-black-red means 1, 0, then times 10^2 = 1000 ohm, that is 1 kilohm; a gold tolerance band means plus or minus 5 percent. Precision resistors use 5 bands, three digits plus a multiplier, for finer values.

Reading the code by eye is a basic bench skill, but it is error-prone: bands can fade, brown and red and orange look alike under poor light, and which end you start from is not always obvious (the tolerance band, often gold or silver, usually sits slightly apart at one end). When in doubt, do not guess; measure it with a multimeter on the ohms range. The colours are a convenience, not a guarantee.