You would have to recode the skin files with variables and use an 'include' file
So in your skin file instead of the font color reading 255,90,0 it would be changed to #R#,#G#,#B#
You would create a FONT_COLOR.txt or something for an include file that would contain this
That way all the font colors, in the entire skin, can be controlled from the one FONT_COLOR.txt file
You can look at the shock v6 skin for example, it has this capability.
After that was done, you would need some sort of trigger to change the FONT_COLOR.txt values when you hit the gamma button, could be done with an environment variable.
Hope this helps some, I can tell you its not going to be as easy as just modifying a line or two of code. You are really going to need to read skin definitions.txt and do some digging in the code.
What skin are you using?