Well, you've never specified which button(s) you want to recolor, but assuming they inherit from the UIPanelButtonTemplate, they don't use the normal texture and pushed texture. Instead, they have separate left/right/center texture regions (to accomodate different widths without distorting the texture too much) and the textures are swapped OnMouseDown and OnMouseUp.
See:
http://wow.go-hero.net/framexml/1644...mplates.xml#18
You'd need to HookScript each method that sets textures, and set your own texture in the post-hook, or just overwrite the relevant methods (though you'd need to be careful to avoid tainting or breaking anything, so simple overwriting might not be an option for some buttons).