Document how to control color mapping

[GoGoCarl]

See this issue for context:
#68

I have ODK field data coming in with color information attached to the field values, i.e. Amenaza^black. Does MapFilter handle stripping out the color information there, or should it be pre-filtered? I'm assuming the latter, but in that case, how do I provide the color information back such that this filter label is colored black?