You can target two blue-and-white multicolored cards by choosing one as the blue card and one as the white card.

You choose the color each target must be when you choose it as a target, so if the card you targeted as a blue card is red when All Suns’ Dawn resolves, the card is an illegal target, even if the spell doesn’t target another red card.

All Suns’ Dawn can have from zero to five targets, one for each color.