The answer might also depend on the color choice of the rest of the game, but a good idea would be to pick two characters that have high contrast with each other, so it’s easy to tell them apart.

This is what Sega did on the original Sonic 2, where the two characters, Sonic and Tails, have colors with high contrasts (blue and orange).

Given a choice only from the colors you’ve shared, I’d pick the purple (3rd from the left) and the blue (4th from the left). But other combinations can also work.

As someone who's colourblind, I'd struggle if you mixed 1 and 3, 3 and 4 or 4 and 6.