Color schmes are XML files in the Plist format and have a .tmTheme extension.
You need to understand scopes (essentially, named regions in the text buffer). To target a scope in a color scheme, you use an entry like this:
... which makes your eyes bleed after long exposure to it, by the way.
I don't know why name (key) + (string) pairs are needed, but it seems all entries have it.
scope (key) + (string) pairs determine what you are changing (the scope "comment" in this case).
settings (key) contains the actual "color information". In this case, the scope "comment" text --or "foreground" (key)-- will be highlighted with #75715E (and "comment.single.line", "comment.multi.line", "comment.foo"...) .
You can read up on scopes here:
sublimetext.info/docs/en/extensi ... xdefs.html
You can find sample .tmTheme files here:
Packages\Color Scheme - Default
The "Packages" folder is explained here: