Congratulations on your new brood of babies, sir!
Now that your pair of Brazilian seahorses (Hippocampus reidi) have begun breeding, you can expect them to churn out another brood of babies about every two weeks or so with clocklike regularity, so as long as conditions in your aquarium remained favorable, you will have plenty of broods to work with and refine your rearing methods.
Keep an eye on your H. reidi for the next day or 2 — it’s very typical for a breeding pair to remate shortly after the male has delivered his latest brood, so you may be able to witness the copulatory rise and egg transfer if you are lucky enough to be at home when it happens.
In my experience, newborn seahorses are typically very pale in coloration at first — often almost white when they are expelled from the pouch — but usually gradually darken to a drab brown or black discoloration over the next several hours. This does not mean that they are destined to be black or brown as adults, however, since the fry often may not reveal their true coloration until they have grown a bit and become juveniles.
I would not expect your yellow H. reidi to breed true as far as the coloration of their offspring goes. By that, I mean that you shouldn’t necessarily expect 100% yellow offspring from a yellow male and a yellow female. Hippocampus reidi has a number of color phases, with brown or black being the most common background colors, followed by yellow, orange, and red color morphs in order of decreasing frequency. So the newborns may reveal a mixture of different color patterns as they grow in the weeks and months ahead.
Best of luck with your first attempt at raising these challenging babies, Nova!