The yellow you see is the last color from bruise resolution. Red to green to yellow to gone. When you couple gravity to the lymphatics of the face, you typically see things in the lower lids even though the procedure was forehead or upper eyes. All normal processes, all going to resolve quickly. And as one of the other docs posted, pretty uncommon in the forehead to bruise. So you don't have to assume it would do this each time in the future if you like what you see from the injection.
This sounds like the end result of a bruise, but is hard to determine without a photo. I’m curious as to if you also had your crow's feet done or just the forehead as bruising in the crow's feet area isn't uncommon after Botox. Gravity also tends to make things pool under the eyes and that could also be the cause. Avoiding aspirin/ibuprofen right before and for 10 days prior to Botox injections will lessen the chance of bruising. “Dr. D”