My stuff is mostly black. My warm water suit has purple highlights and actually looks quite nice, but I bought it because it was the best fit.
I'm considering upgrading my cold water suit after the holidays to a one piece, but the reason I have black on black is actually because the cheap jane/jacket combinations seem to only come in that color and I am a cheap soul. My fins are basic Volos with the gray stuff, designed to look very unassuming in water. I got sick of mixing my fins with the fifty other people that have the same style, so I wrote "Your Ad Here" on the back of each before going to the Philippines, and though I got lots of jokes, I never mixed up my gear!
I do wish I'd gone with either the blue or red dive rite wing though. A LOT of people have the Transpac 2 and I can't find a way to mark it that clearly, and in the PI, I actually got about thirty seconds away from swiping the gear of someone from our dive shop. D'oh!
There's been some research on which colors sharks seem to go for. If you're a silhouette at the surface, they're probably not going to see your colors that clearly, and I've heard stuff like yellow increases contrast, so they may be marginally more likely to "test bite" you to see what you are. I figure my chances of getting tagged are slim, so I try to make my 'danglies' yellow for easier spotting by buddies. I'm tempted to paint my tank yellow, since I had a buddy dive a yellow tank while we were in bad vis, and I NEVER came close to losing sight of him.
I like some color variation too because when you're diving cold water with black suits and hoods, it can be difficult to keep track of buddies. When I was diving bad vis with a different buddy in a class group, I was with a DM and my buddy, and we actually ended up TRADING buddies because we had two people in black with black fins, and the DM looked back, looked concerned, made a "buddy up" motion to me, I gestured to my 'buddy', looked harder at the eyes and went "uh oh", which apparently was exactly what happened to the other DM/buddy group. D'oh! See, a pink snorkel or yellow fins would have prevented that!
Also know a guy that dives a red (pink at depth) drysuit. He gets teased relentlessly about it, but you would NEVER lose him!