Re: vomiting yellow stuff
Yes, what she's throwing up is yellow bile. Some dogs vomit in the morning (or in the wee hours of the night) if it's been too long since their last meal (I have to feed two of my three dogs a small handful of kibble right at bedtime to avoid this).
Did she eat any grass this morning? Grass will definitely do this too. One of my late-night kibble snackers is a grass-barfer. In the morning, after she potties, the first thing she'll do is head to the area right near the fence where the grass tends to grow longer (because we don't weed whack every single time we mow.) I have to supervise her like a hawk. Once she has breakfast, everything is fine.
Grass, BTW is not good for a dog's stomach. I was once told it's like eating shredded newspapers. Won't kill you. But not particularly something you want to be doing. Any nutrients it might provide is offset by the fact that if eaten on an empty stomach, you're likely going to see it again anyhow.
If it's either of these, then you have a dog that you need to supervise, either with a snack at bedtime and/or in yard first thing in the morning. And I wouldn't stress too much about it. If you can't pin it down to either of these very specific situations, I'd be heading to the vet's office.