To person who is colorblind and mixes up red and green, what would those two colors look like to them?

Shades of grey. It is hard to know exactly what a color blind patient perceives when looking at these colors, but most say they are shades of grey. Color blind individuals have a smaller pallette from which they perceive colors and do not know the entire range that we see. This makes it difficult for them to explain exactly what colors they do see.
I see all colors. As an ophthalmologist who is red/green color blind or, more accurately, color confused, i can assure you that i see all colors but some tints of red and green can be a little difficult to tell apart. Things for me do not appear gray at all.