Deltarune Chapter 5 Part 7

Personally, I think the blue/pink voucher is supposed to represent Kris' colors and the green/yellow is Asriel/Ralsei colors. The purple one is for Susie. Ralsei wanted to drink something different than what was assigned to him as a metaphor for going against the prophecy. This is why Ralsei was acting as if Kris would have a problem with them choosing the blue/pink drink; it was the drink assigned to Kris.