I'm trying to find out because I'm making a mod for Minecraft and I'm trying to keep the regular brightness variations
Color is a pretty complex subject.
There are several aproaches to modify colors in a file. For example you can vary colors using the "Color > Hue and Saturation" tool. Technicly you are not changing brighness if you only change Hue, but for example if you previusly had an yellow texture and change the Hue to a Blue it will be "darker" to the human eye. You probably have to adjust the values with the "Color > Levels" tool.
So it is more a perceptual problem, rather than a technical one.
Probably you need to work with a screen capture as a reference, and make your tests comparing this screen capture.