1. (as you mentioned) colors don't match; could 
be a correction though.

2. doesn't match the scene

Watch the episode from where the cel came again. 
Look very, very carefully with frame-by-frame or 
slow motion through the sequence. Pay careful 
attention to details such as the placement of 
fingers, hair, shading, eyes. Everything. If your 
cel doesn't match, it's possible it's a fan cel. 
I've seen cels so good they could easily pass as 
originals. Scary.