So long as they are of similar size and all healthy and the tank is big enough and ready for them... their colours are up to you.
Personally I enjoyed having different coloured discus lol, was certainly a rainbow tank! I had a Red & White, Blue Cobolt, Red Turquoise, Blue Turquoise, Brilliant Turquoise, Red Melon, Penang Erruption, 3 different coloured Pidgeon Bloods, White Butterfly and I forget the names of the others!
Most important thing when buying new discus is to worm them in my opinion, if you can buy Kusuri Discus Wormer, it might potentially prevent you losing your discus in a few weeks/months time. The stress of moving to a new home means their immune systems will be low and their natural parasite load will multiply beyond control. Best to treat them after their first week.