I’m going to start a gig on editing the CSS code of a site to change its design. So if I created the gig using edited versions (colors only) of popular sites such as facebook, IMDb, Linkedin, etc as examples of design changes, is it still copyright infringement? Will it be a problem for me?
That’s a question to ask a lawyer - I doubt anybody on the forum has sufficient legal training to be able to give you a helpful answer.
It depends what you do with the final delivery. I don’t understand how that would help you sell services though. You should make your own CSS or at least create some templates for yourself to use with clients.