All parrots, as all faders do, have the ability to change the full gamut between deep reddish orange to yellow to white. I have raised blood parrots for years and have had every colour.
Most parrots that are made from midas will change either yellow or orange, and the color that the first one fades to will be the ones the subsequent parrots change to. All the ones I get from wholesale as generic "parrot fry" (i get them at about .5 inch or so in bulk) turn yellow if I keep them alone. That is if I fade them in a tank by themselves they will all turn the yellow colour of the yellow midas cichlid.
if you place two faders of slightly different colors together they will change to match eachother. My theory is that midas cichlids and red devils in the wild differentiate different "tribes" of fish by their colour.
So whenever I fade parrot babies by themselves, they turn yellow. however, as soon as I place these baby parrots in a tank with some of my growouts, which are all orange, they will turn orange. The dark ones will turn slightly more yellow and the yellow ones will turn slightly more orange until they are all the same colour. Conversley, when I add a full red blood parrot to the tank with my orange parrots, they will all get more red and the bp will turn orange.
for a more extreme example, I once obtained a full white pink convict x orange blood parrot cross. I kept it by itself for a while and it was white/pink in my tank. After a week or so I placed it in a 49 gallon with about 15 young yellow parrot growouts. By the end of the day I couldn't tell which one it was because it had turned the same yellow as the rest.
Ergo, parrots are surprisingly adept at changing colors, though they only do so to be the same color as the community. I garuntee that you can find some yellow parrots, but you may have to get the least red brown parrots you can from petsmart in order to have them change to yellow for you. You could also get yellow parrots by buying a group of unfaded babies and growing them out with yellow cichlids such as gold severums or a docile yellow midas.
So basically, you don't see yellow parrots any more because orange and red are more popular. red is the "most desirable" colour in most breeder's minds, and most people have blood parrots. So given that there are so many red parrots out there it goes that most yellow parrots turn orange as a result.
Hope that answers your question lol