Just as a note, real world HUDs tend to be a single colour: their purpose is to provide basic essential information, not a graphically intense video that could distract the operator, not to mention that if the background changes you could lose some of the information from the screen.  That's not likely to change in the future.

That being the case, you don't need two maps: a single greyscale map is all you need.  Use it as a transparency map, and then use it to drive to drive colour, either setting the diffuse colour to what you want directly after plugging in the map, or inserting a simple_color node between the map and the diffuse (and of course setting the diffuse to white).

If you wanted a multicolour display, so long as there weren't too many and they were separate elements on the screen, same thing.  You use a process similar to silk-screening: one map for each colour.  Use blender nodes as appropriate.

The advantage of doing it this way is that if you change your mind about what colours you want displayed, you don't need to go back to your graphics editor to change your image.  You do it right in Poser.