They're just house/clan colors or colors of a specific Mandalorian organization like the Nite Owls. Since the Children of the Watch are descendants of Death Watch, I'd assume Paz's blue armor is a shout out to them. Or maybe those are just house Vizsla's colors. Pre Vizsla wore blue as well.
Din didn't have a clan at the start of the series so his armor is unpainted. Or maybe he just likes the color of beskar.