A resistor is a discrete electronic component designed to oppose the flow of electrons in an electrical circuit. This opposition to electron-flow is called "resistance," which is measured in a standard unit called an "Ohm." Most discrete resistors are marked with a series of colored bands that will indicate the value of the resistor in Ohms. This convention is known as the "Resistor Color Code." What follows is a guide to using and interpreting the Resistor Color Code.

On any given resistor there will be a series of colored bands including the colors of the rainbow, black, brown, and a metallic colored band of silver or gold. Collectively these bands will indicate the value and the tolerance of the resistor. (The tolerance is the percentage by which a resistor may acceptably deviate from its marked value.)

Before I explain how to use the Resistor Color Code, I will first define what each of the color bands means. It is important to note that one begins not with the metallic colored band but from the other end of the resistor, reading the colored bands in sequence from left to right. Each non-metallic colored band, except for the final one, will be assigned a numeric value. The final non-metallic colored band will serve as a multiplier that determines how many zeros are to be added to the previous numbers. And finally the metallic band will specify the tolerance of the resistor, as stated above. I will first define the meaning of the various color bands below, and then I will illustrate with an example. Below is the standard interpretation of the color bands on a resistor:

0 - Black
1 - Brown
2 - Red
3 - Orange
4 - Yellow
5 - Green
6 - Blue
7 - Violet
8 - Grey
9 - White

If the color band is the final non-metallic color band on the resistor, the color band is a multiplier and will have the following interpretation:

x 1: Black
x 10: Brown
x 100: Red
x 1,000: Orange
x 10,000: Yellow
x 100,000: Green
x 1,000,000: Blue
x 10,000,000: Violet

And finally the metallic colored band will have the following interpretation:

Gold: +/- 5%
Silver +/- 10%

I shall now illustrate the use of this color code in an example. Suppose that one had a resistor with the following color bands in sequence from left to right: yellow, violet, orange, and silver. Starting from left to right, the first band would have a value of 4, the second band would have a value of 7, the final non-metallic colored band is a multiplier of 1,000, and the metallic silver band would have a value of plus or minus ten percent. Putting this all together, the first two bands have a value of 47, which would then be multiplied by 1,000, for a final value of 47,000 Ohms. Since the resistor has a tolerance of 10%, the actual value of this resistor may be higher or lower than 47,000 Ohms by 4,700 Ohms or less.

The above method for determining the value of a resistor using the Resistor Color Code generalizes and can be used to determine the value of the majority of discrete resistors that are commonly available in electronics.

Source: http://www.associatedcontent.com/user/75832/zachary_fruhling.html