Red green color blindness is the most common form of color blindness.

To someone with that form of color blindness these shirts would be different shades of the same color (think, dark red vs light red).

I have one coworker with red green color blindness that said that peanut butter and zucchinis were roughly the same color to him (Oh, it's also more common in men than women).

(disclaimer: not an eye-ologist)

Check out the graph of the light that is picked up by the 3 types of [https://en.wikipedia.org/wiki/Cone_cell](color cones) (top graph), turns out the wavelength picked by the green cones and the wavelength picked up by the red cones are actually pretty close, not opposite at all.

The brain normally figures out which color it sees by calculating how much light each type of cone receives. Red green blindness means the brain fails to interpret this distinction. A green light actually still activates the red cones, and vice-versa, but there's less of a reaction, and the brain can't compare that to the excitation of each type of cones correctly, so it just thinks that only one type of cone was excited, only a bit less. That's why one color can be mistaken for the other, only more dull.

Apparently red/green colorblindness is the most common. Also, if you pick a dark enough shade of green and a light enough shade of red, they'll appear the same in black and white. Someone might assume that the darker is red and the lighter one is green and might get it right most of the time, but looking at just one thing by itself you would have nothing to compare to.