You can tape off the areas that are not suppose to be black (use blue painters tape, comes off easier) then paint them the color he wants. I did this with my airbrush on my Orca cake. I would steam the fondant after to make it shiny, be careful not to let it run.
Color will not show up on black.
Thanks for the help! Last night I tried Wiltons sparkle gel on a piece. It gave me exactly the sparkly, slight color. Just painted on! Thanks again!
This is probably not the answer you are looking for but:
If airbrushing, I always wrap the cake in the lightest color and then paint downward to the darker values. It is a lot like Water color painting.
You can hand (brush) paint it with gel color (gel is thicker and more opaque) but the fade off (spray paint look) you are going for will not look right or good.