Is he throwing up first thing in the a.m.? If so, it could be hunger pukes; upset tummy from extreme hunger. This is normally a yellowish color. If he's throwing up in the a.m. before breakfast, you might want to give him a bedtime snack (divide up his usual portion to take this snack into account), so that he doesn't have to wait so long between feedings.
If this is not the case, a vet visit may be in order, as mentioned above.