When you save a project, Sublime Text will create a .sublime-workspace file. In this file, there is an array of buffers, and for each buffer there is a color_scheme property. Th
Expanding on the answer given by Tot you can do this for all views in all windows that are open by using a nested list comprehension (remember this is Python so we can be pretty flexible):
[ v.settings().erase("color_scheme") for views in [ w.views() for w in sublime.windows() ] for v in views ]
This way you don't have to run the command in each tab individually.