Sometimes you may not be able to find the color you want in a paint designed specifically for a one-step application to plastic. Most modern single-stage acrylic enamels (like those in spray cans) have enough plasticizer to flex along with Segway fenders and trim without cracking. Exceptions might be some of the textured paints that rely on very thick films to create a particular surface effect. For example, Rustoleum's 7213 Hammered Silver paint for metal would probably create too thick of a paint film to flex without cracking (but they do make a Hammered paint for plastic that might work).
When I painted the adjustable fenders and wheel inserts on my i180 with lacquer-based color-shifting paint, I used the following steps:
1) Wash the surface with diluted automatic dish washing detergent and hot water, then dry with a soft cloth. If there's dirt, wax or oil present, you don't want to work it into the surface while you roughen it in the next step. Unless a plastic part was specifically cleaned or painted by a manufacturer, most molded plastic parts are supplied to you with mold release (a wax) on them. I recommend using disposable nitrile or vinyl gloves (not latex) to handle the part from here on out to avoid transferring body oils to the part. You can usually buy these gloves in boxes of 50 or 100 at large drug stores.
2) Buff the plastic surface with 0000 steel wool to roughen the surface. The microscopic scratches will not show up in the paint but will dramatically improve adhesion.
3) Wipe the surface with Klean-Strip's Prep All (or another purpose-specific wax and grease remover). Prep All is about the cheapest degreaser available and seems to work pretty well ($13/gal up here, others as much as $40/gal). Denatured alcohol would be an acceptable but less desirable substitute, but avoid isopropyl alcohol - it has too much water in it.
4) Lightly wipe the surface with a painter's tack cloth. This is a wax-impregnated cheese cloth that is compatible with paints . . . auto parts or paint stores carry them. Don't skip this step or you may end up with lint or dust under your paint.
4) Spray on one coat of Duplicolor's Adhesion Promoter. See
http://www.duplicolor.com/products/adhesion.html. This product is usually available at the larger auto parts stores like NAPA, and is specifically designed to help paint adhere to plastic.
5) Apply one thin coat of primer if necessary for the color being used next.
6) Apply thin color coats, using as few coats as necessary to get good color coverage. This is usually two coats, the second applied wet enough to flow out as soon as the solvent flashes off the first coat.
7) If you want a really durable finish, I recommend applying one or two clear coats with something like Duplicolor's DA1692 Crystal Clear. I don't recommend Krylon's clear acrylic, it's just not a very high-quality paint. Because clear acrylic doesn't contain fillers, it is much harder and more durable than color coats of paint. The clear coat will keep your paint job looking good much longer, providing scuff resistance that the color coats just won't have.
I haven't mentioned wet sanding here. That's probably beyond what's required to paint your Segway, but I can provide more information on that if you're interested.
Glen