Imagine this... You're hanging out with a strange person who looks like an anthropomorphic lizard-squid hybrid with cuttlefish eyes and a tacky wardrobe, and they ask you something like this: "What do you think? Does [insert made-up color name] go well with magenta?" You sadly can't give a proper answer because the shirt is of a color humans can't see and as a result looks like a yellowish gray instead.
So yeah... This is my idea of conveying an impossible color for the Color out of Space challenge; displaying a character wearing a shirt that would look gray to humans (and I'm pretty sure you're a human lol) as opposed to whatever said character sees because the character's eyes possess more color cones than human eyes. I hope it makes sense.
16 colors: 15 + transparency