Yellow is non red and recessive, so you are right there If spawned to red, you will only get red. Yellow shows up in the second generation as both fish (if from same spawn) will be carrying the recessive non red gene. As the dragons carry a red gene, when combined with the yellow non red, I should only get red showing up in the marble pattern. The second generation will show yellow, as the siblings will both carry non red...
OK, I just confused myself. I hope I made sense :D