A Color is represented by an eight digit Hex Value, like
# AA BB CC DD
where AA is Red, BB is Green, CC is Blue and DD is the alpha channel (opacity).
An RGBW Color is represented by a ten digit Hex Value, like
# AA BB CC DD EE
where AA is Red, BB is Green, CC is Blue and DD is the alpha channel (opacity)
and EE is the additional White channel.
Due to the fact, that you can enter the hex values, when you only want to change a single channel, here is a chart with a lookup table and a small calculator.