Find yourself some red transparency paper, or any thin film that you could either send through a printer as a sheet, or tape to a piece of paper to send through a laser printer. Print the reverse image of the gauge, then any light showing through the uncovered transparency will appear red. All it takes to put different colors on the same gauge is a little creative work with an exacto knife. If the laser printer doesn't completly black out the wanted areas carefully run it through a second time, or use a paint marker or hobby paint to go over the bulk of the black area to avoid any spots that show through. It's really only limited by your imagination and ability to use simple tools. With good blackout paint and some creative light blocking on the other side you could do some pretty sophisticated looking gauges, definitely worth the time.