Are they really Color Blind? Another Grandpa Mystery

This riddle is a "thanks" to user477313 for his Grandpa Puzzle.

" I just got back from visiting my cousin Bob" said Grandpa

" There at a beer party I met two of his friends who claimed they were completely color blind. So I showed them three cups. One was brownish, one was grey, and one was bluish grey looking. Pretty close actually.

I asked them do they see or recognize any color?

And they both picked the Grey! Not the brownish, not the bluish but exactly the Grey.

Dang. Can you explain this son?"

I think

Bob's friends recognized one of the cups and, despite being colorblind, could deduce the colour based on its name.

It was

the Grey Cup, the championship trophy of Canadian football.

There are probably not colorblind:

If they were, all cups would look grey to them and they would pick one at random. Them both picking the actual grey one could be sheer "luck" (1 in 3 chance), but more probably they were trying to pass for being colorblind and forcefully picked the "wrong" cup.

If they are indeed completely colour blind, they will only be able to see in shades of grey. So whichever colour they pick is grey and who are you to say it's not. Colour after all is subjective. Someone may see what you perceive to be green as what you perceive as red but as everyone has told them their whole life it's green they will call it that and will never know any different as noone can see through their eyes.

If you are actually saying they picked the same cup that is different but you said they both picked grey so they could be different mugs which they both see as grey (in fact to them all the mugs will be grey).

The grey cup was the only one filled with beer, of course they picked it.

The grandfather is colorblind as well, just a different type than the others. So what he sees as grey is likely a color the other two can see.