完成前とずいぶん色が違うじゃねぇか!
In this sentence, what is じゃねぇか?
じゃねぇか is just a colloquial form of the tag question じゃないか.
This doesn't only happen with じゃない > じゃねえ, but generally /ai/ > /ee/, like
きたない > きたねえ
やばい > やべえ
(食{た}べたい = ) 食{く}いたい > 食いてえ
As in the other answer, this is extremely informal and in the wrong context can easily be considered plain rude.
Xと違う = to differ from X
完成前 = before completion
Edit. For completeness, there's also