That's sound like what happened to me when I was sick.
I was in the hospital (something unrelated to throwing up) for about a month when I was 14. I didn't eat until maybe the last week there because I kept throwing up (I was being "fed" through the IV). I threw up all sort of colors with an empty stomach but it usually ranged from orange to green/yellow.
With an empty stomach, you just get weird colors because of your stomach acid reacting to whatever is left along the lining of your stomach. If you throw up too much, you also will see black flecks.
He may just have a slight case of food poisoning. Monitor him, but don't stress too much. And if you can give him some dairy to releive the acidity a bit.