I am looking for a movie featuring an arts student who meets women, paints them, then kills them. Definitelly color. I'm almost sure it's an American movie. I remember that the painter was seeing one girl during most of the movie and that she found out that he was a killer in the end of it. He has a bunch of sketchbooks with his victims' names and paintings of them.
I think you're looking for this one - Spiral (2007). The storyline matches your description.
Mason might be the painter you're talking about,
and the story goes like this,
Try this to get more information about this movie.
Here is the trailer of Spiral which matches most of your description.