I've been interested in this myself, especially as one of my players has (semi-inadvertently) brought Tiamat's dead sister into play as a (possibly major) plot element in the campaign I'm currently getting started.
My thoughts so far:
The major changes that need to be made are to the Breath Weapon and Immunities. Orange, Purple, and Yellow Dragons do not get elemental Immunities, so neither should the half-dragons based on them. Half-Oranges should get either poison immunity or water breathing. I would lean twords Poison Immunity just for the sake of uniqueness. Their breath weapon can either have the same effect as the dragon's, or else merely do concussive (normal) damage. I'm inclined to simply have it deal damage in a line, if it works like the full dragon's, then the damage should probably be reduced (to, say, 4d8). I think that the delay in the effect is supposed to compensate for the potential to deal damage over a much larger area, but as there is no means of removing the "slime" once hit I think it may be a bit much. The Half-Purple gets no special immunities, but has the choice of breath weapons each day. I would allow the blinding effect as a 30' burst with a 30' range, but reduce the effect to 1d4 rounds of blindness followed by an equal duration of dazzled. Also, the cone of force damage seems appropriate, it's better than elemental (or normal) damage, but this is compensated by not getting any immunities, IMHO. For the Half-Yellow, I would give water breathing in place of any immunities, and give it the same breath weapon in a 30' cone, dealing 3d6 damage instead of 6d8 but with the other effects as normal. I don't think that would be too overpowered as a once per day ability. Also, the Yellow doesn't have wings, but that seems more of a cosmetic difference than anything else.
