Add way to change colors

[SHiLLySiT]

The Playbit shader currently uses an off gray and white to reproduce the look of graphics on-device. There should be a Playbit API to allow changing colors to support alternative palettes without having to modify Playbit.

There's also some old cruft: the color constants defined in graphics aren't actually used - colors at some point were moved to hard-coded values in the shader.