On of the things to think about is contrast. If you want to have the tins stand out you go with a contrasting color. I got my tins painted a deep blue with light flakes in it. I think it contrasts the frame and really looks good. Now if you are going to do the frame and keep the tins, I would suggest you need to do more than just the frame. Fork sliders at the very least. Hand controls, handlebars,headlight bucket,etc maybe.
Everyone is saying black but I would vote against it. Everyone's frame is black (although you are going to put flake in it). Look for a deep dark color.
My useles opinion.....well, since you asked, the absolute HOTTEST bike I EVER saw, was a Ducatti with a red frame and black tins. So, I say powder it red, then when you realize it just looks good, go ahead and powder the tins jet wet black, add chrome and, ooohhh-la-la !!!!!!!