TMI-1month old keeps spitting up a ton

My baby has been having an on going tummy issue. We had to switch formulas from enfamil neuro pro to the gentleease neuro pro. So far no changes, I know it takes time. But my baby has been spitting up a ton all at once after every feed. We saw the doctor yesterday and she said give it more time. Although at 1pm he vomited a lot and it was a rusty brown color. I called the on call pediatrician and she didnt seem to concerned as he still is having frequent wet diapers. I cant tell if this looks like blood or old formula in his stomach. The pediatrician said just to keep an eye on him so I havent slept in two days. I'm planning on calling his doctor when the office opens even though we were just in yesterday. Has anyone experienced this? Does it look like blood? Any other advice? FTM