First, there is a fairly uncommon Moto Guzzi, the Nevada. It's possible its one of those. If so, I'd suggest a segmented blank that's Red/White/Green for one half, Blue and Silver for the other half. Those are the colors of the Italian and Nevada flags, respectively.
If it's a Harley Davidson, simply go with the three HD colors: Black, Orange, and somewhere between gunmetal and silver. Or chrome.
Another possibility if the bike is a cruiser is to turn the pen out of Aluminum. The pen will therefore be a Billet Pen. If it's a custom "chopper" (possible), then your best bet would be to turn a blank somewhat undersize, take it to a good nailbrush painter, and have her (most likely) paint the pen with a paintjob that echoes the paintjob on the bike, then overlay the paint with resin, turn it back down to size (without, of course, screwing up the paint job), assemble, present. While it would likely be in the spirit of a custom cruiser to make the pen uncomfortable to use, why rub it in?
One more option for the cruiser crowd would be either a snakeskin blank with chrome, or a leather blank with chrome.
Next possibility is a sportbike. If its an Italian bike, see above for the Italian colors. If it's a Japanese sportbike, each Japanase maker has a signature color. You could have a blank made in that color, and then turn the pen. 'Tis a similar situation for dirtbikes. Signature colors. Another possibility for a sportbike rider is either a carbon fiber pen, or figuring out some way of making a pen using stainless braided brake line. Last possibility for either of these is an anodized aluminum or a titanium body pen, as both materials are used frequently when customizing sportbikes and dirt/motocross bikes.
Next up come the Wings and other luxury tourers. Comfort pen all the way, with chrome accents, and a lit'l CB antenna. Or a Gentleman, after all, "you meet the nicest people on a Honda."
Last are the real motorcycles. Well, "near real", the BMW touring bikes. For one of these, it's gotta be a gadget pen. Make it pricey.
Finally we come to the real motorcyles. They are farely rare, as the number of folks who have enough wisdom to recognize them is low. STeeds. Fast, comfortable, nimble, reliable and long legged. Did I mention fast? Most STeed pens would best be in the shape of a rocket booster, except for the blue ones, which don't need boosters, 'cause they're already "oh crap" fast. Anybody with a red STeed would probably need two pens, 'cause that's the slowest color, so it needs two boosters to keep up with the blue ones. :tongue: Lanman's right about making it a really bright color, and a little flashing LED in the cap would be nice touch. A Polaris or Atlas (like STeeds, both are intercontinental missiles) would be a good kit.
Oh, and if it's a STeed pen, go rollerball or twist, not fountain. STeeds cover so much territory that the rider would run out of ink making notes on his maps too often. Gotta keep the refill regimen simple as possible.