to be fair, he never said he couldn’t tell he was blushing, he just said he didn’t care, right?
my pa had it but it was more bad like reds and greens were light to dark greys
at times it even mess with blueish green colors as well or more brownish red tones
so to the dude in the story have little bit of it
he may see red as a pinkish or light grey as well?
You’re right, you don’t know enough about color blindness.
Color blindness is a wide spectrum, so it’s different for every person that has it.
For some colorblind ppl, blush is likely not going to be distinguishable from human skin. For other colorblind ppl, it will be.
There is no one way that color blindness works, it all depends on the person.
But it’s implied he has red/green color blindness (deuteranopia) which typically does not impact how light or dark things are. Blushing would probably make him darker (though I guess it’s possible he blushes in the exact same shade as his skin). If he was unable to distinguish shades (like depth of color) he would have a tough time advancing in art where shadows and light are really key.
I don't know enough about color blindness but I think even if he can't see red he would see it like another shade of color so he should be able to tell he is blushing