I want to manually edit my color scheme, but I can't seem to find it anywhere.
There are two types of gtkrc files:
(1) "System-wide" theme files which reside in the location mentioned by hasen
(2) Your own customizations, which are stored in ~/.gtkrc-2.0 (Note that the 2.0 is important)
Usually there is no need to edit the system-wide files, and user customizations should be in the home directory