OR does not guarantee color. There is really no way to guarantee color, because although seahorses have a base color, they can and do change color. If you want yellows and oranges your best chance for those colors is to go with Sunbursts, but as I said there is no guarantee. I had a yellow Sunburst change to platinum and her partner turn a light pale tan color. I also had a blackish brown Mustang turn and stay bright red. You just never know. You can add lots of colorful decorations to your tank to help the horses retain their brighter colors. Reds, oranges and yellows seem to work best.
Sunbursts tend to be a bit smaller than Mustangs. IME the Sunbursts are also for the most part smoother than the Mustangs which can have a bit more texture.