Resistors are used to control current flow in a circuit, and are rated by their resistance value and their power rating. Only the resistance value is marked. The resistor's power rating is determined by its size.
Regardless of their power rating, resistors are small, so identification by numbers is impractical. To identify its value, each resistor is marked with four or five colored bands. Each color represents a number value, so by reading the color values it is possible to identify its resistance value. The color bands are placed so that they are set close to each other and biased to the left. The last, or tolerance band, is spaced further apart.
The chart below explains resistor colors. It is good practice to memorise the various colors and their respective values.
Resistors are manufactured to a tolerance of 1, 2, 5 and 10%. A resistor with a tolerance of 1% will have an actual reading that is within 1% of its marked value. Resistors with a tolerance of 1 or 2% will have five identifying bands. A resistor with a tolerance of 10% will have an actual value that is within 10% of its marked value. Resistors with a tolerance of 5 or 10% will have 4 identifying bands. The tolerance band is spaced away from the value bands.
If a resistor has four color bands, the first two bands indicate the first two numbers of the resistor's value. The third band represents the multiplier value and the fourth band indicates the tolerance. The image below shows a resistor with colors, Orange, White, Yellow and Silver. It has a value of 39 kΩ with a 10% tolerance. (Orange - 3, White - 9, Yellow - 10000, Silver - 10%)
A resistor with five color bands adds a third number value, so in this case the first three bands indicate the first three numbers of the resistor's value. The fourth band indicates the multiplier value and the fifth band indicates the tolerance. The image below shows a resistor with colors, Orange, Orange, White, Black and Brown. It has a value of 339Ω with a 1% tolerance. (Orange - 3, Orange - 3, White - 9, Black - 1, Brown - 1%)