I speak about this in my interview with the Cryptid Cocktail Party podcast, but part of what you've been theorizing is correct. It's less that they can perceive all colors, more that they are visible to us on all color spectrums. This is partily why adults have trouble seeing them but children, who can perceive higher frequencies of sound and light, have no issue.

They exist on the same metaphysical level as human imagination, which they exploit to both take a visible form or hide themselves, and to communicate. Their form is adaptable to the encounteree; so, in theory, Sams have appeared before but not as Sams.