I've heard some dumb questions before, but this is the killer. Of course it has to be red. Everyone knows that, don't they? Okay, okay, so I'm a little biased.
Seriously, though, I do think that red is a great color to use for most of the bike. But it is nice to get some other highlighting colors in there. I'm fond of wide, angled stripes and lightning bolts, and stuff like that. I'm not fond of gray, but black and white offset the red nicely. If I were doing red/black, I would more likely use red on the higher surfaces and black on the lower ones. I think that would look better and hide the dirt better.
That being said, it's hard to argue against black bikes. A little gray would look good on a black bike. Look at the 2005 Ninja 500R for a great example. They also have some white stripes to bring it all together. Kawasaki got it right on this one.
You asked for opinions, so I gave you mine. But pick what you like, and you'll love it forever. That's what really matters.
Curt