When I handed off the idea of what Yamiko-chan would actually look like, I pointed out how the shape of the headlights would be relative to the shape of the eyes, how they narrow down towards the center of the face. Then the eye color came from the fact the dashlights were red. I know, those two aspects don't have much in common with each other, but still, I really, really like how this turned out. Hair color is relative to paint color. The same formula was applied to Ai-chan, with similarly excellent results.
As for the name, I derived that from how her paint wasn't completely black, but almost, and her interior was mostly black as well. A very dark car, and very new (when I bought her). Hence, Yamiko, which means "dark child". And when really pushed, that darkness could show in her personality as well.