Resource Replacer (Alpha)
Replaces textures and colors of buildings based on a "Resource Pack" (a separate mod that contains textures and color settings).
How to extract and modify textures: http://community.simtropolis.com/forums/topic/68595-what-is-more-important-a-texture-replacer-mod-or-a-custom-bridges-mod/?do=findComment&comment=1593882
How to modify color variations & how to create a Resource Pack Mod: http://community.simtropolis.com/forums/topic/68595-what-is-more-important-a-texture-replacer-mod-or-a-custom-bridges-mod/?do=findComment&comment=1603084
You can reload textures ingame using the key combination CTRL + J (note that this will slowly eat up your RAM because the old textures are not destroyed, but it should work hundreds of times before it becomes a problem).