More Graphix
If you still haven't seen enough designs, just take a look at some of the following:

The VW Bus was completely disassembled, Matchbox Premiere "Porsche" front wheels were added, the cockpit was color matched to the exterior scallops, and the body was painted with Marbleizer over Majik Blue pearl, highlighted by metallic green scallops.
The Beach Blaster rides on Vintage Red Baron redlines and was painted Pavo purple metallic and Snowhite pearl.
The Rigor Motor body was extensively smoothed and then shot with black, marbleizer, and kandy orange (in that order). It too rides on new redline wheels. All paints courtesy of House of Kolor.

Here's a simple design used to add some color to a Custom Cougar body. As it turns out, the recipient decided he'd prefer just solid yellow, so the body was later stripped and repainted. The paint lines were created using tape that was cut with special art scissors available at Wal-Mart, MJDesigns, and other craft stores.

Here's an old Poison Pinto repainted in basic white and decorated with custom water-slide decals. New redlines were added and the headlights were detailed.
The customizer is really only limited by his or her imagination when it comes to creating unique designs. Some have made cars with lightning bolts painstakenly painted on, incredibly detailed murals, multi-fades, and this latest Eddie Van Halen inspired creation from Van Designs (as proposed by Bill MacFarland):

Once you've got the basics down the sky, as they say, is the limit!