Well, I see them as cyan, yellow, etc when I play.
I guess yellow doesn't show as good on the tactical view though.
It probably depends on your monitor and video settings, whether you can see them or not.
I would have to say, black could hide on either view! (on the lava terrain by the volcanos, its black either way)
Editing colors: If you're interested, extract color.bmp from maps.vol and look at its palette.
The colors repeat in multiples of 24 colors each. If you want to change them, just change the correct 24 consecutive indexes.
Then save color.bmp in your game dir, and play with the new colors