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.
I don't know when the last time was, weeks ago at least. He hadn't eaten much all day. He did eat later and doesn't act sick. I guess it could have been grass but I saw none in the mess. I'm setting up him getting neutered so I'll ask his vet about it.