If he's half red and half silver how would the attacks work? Since Red Dragons breath fire and Silver Dragons breath cold and paralyzing gas. Would it be like it breaths hot water and paralyzing gas? Also Red Dragons are always Chaotic Evil and Silver Dragons are always Lawful Good, so why would they mate?
He might have a sort of coldfire breath that deals 1/2 fire and 1/2 cold, and have no no actual fire or cold subtype (thereby gaining neither immunities nor vulnerabilities. He'd probably miss out on the paralyzing breath, though.
And I for one am a fan of that little caveat in the monster manual and BoED- that non outsiders with the "always evil" alignment aren't actually evil 100% of the time. Or maybe the red dragon was Sanctified with the spell?
Maybe he was created instead of born? Seems to be a growing trend that D&Ders are taking advantage of the fact that there seems to be a mad alchemist around every other corner when they're making characters.
It all depends on how loose you play the rulebook.
Although this drawing is totally epic, it does beg the question - How would a half red/silver dragon come about? A silver would never do the nasty with a red : That's like Freddy Krueger getting it on with Mother Teresa.