Resistor colour coding is used to indicate the value of any resistor. It was discovered by RMA (Radio Manufacturers Association) in 1920.
The first 2 bands in this encoding scheme indicate the first two numbers whereas the third one indicates the multiplier and the last one represents the Tolerance value.
How to calculate Green Green Yellow Brown (GGYB)
Suppose you are using a fixed resistor which has four bands having GGYB configuration. Select the first band as green from the calculator, second again as green third one as yellow and 4th one as brown. Finally, hit the calculate button and you will get your answer in the second row. The third row provides the tolerance value.
As an example for above case of GGYB, 550 kohm ±1% provides information that the given resistor could have any value ranging between 544.5k-555.5k.