Not as a sole criteria.
If you are "set" on a certain color or pattern.. that's fine, but don't compromise the rest of the package for a pretty color. For me conformation and temperment always come first - regardless of the purpose the horse is going to have to me. (Training is less of a consideration for me... I enjoy training my own... but for most people it would be up there on the list ahead of color)
I waited 7 years to find my cremello Saddlebred colt. He's a stud prospect - provided he continues to grow into a quality horse all around. His coloring was the icing on the cake, and I was specifically looking for a double dilute to fit into my future breeding program. I would love to be able to breed quality, athletic, horses with that added bonus of a golden color. There isn't really anything about this colt I don't like - and that was my PRIMARY concern, so far his conformation is great, his temperment is great, he's easily trained - as time goes on he'll have to continue to prove himself before he's ever bred, but, I will admit that his color played a role in my decision to purchase him.
I have a cream champagne tobiano mare which is a similar story. She has great conformation, an absolutely wonderful temperment, she's smart, been a successful broodmare (in her previous home), and is doing well in training. When she's "finished" she'll be a broodmare for me... but I wouldn't have bought her without the temperment she has, or if her conformation wasn't up to par.