But wouldn't the color of the pieces look the same color as the real pieces? 
Yeah but if you can see less colors altogether then it's easier for some of them to be similar and confusing because there's not as much contrast. Instruction books are often printed carelessly, where the red on the page, for example, doesn't match the red of the actual piece in saturation or lightness. That is more confusing to somebody who doesn't have the "red-ness" to help them. Imagine doing a black and white photocopy of the instruction book and of all the pieces and trying to match them that way.
There could also be quite a difference in the way his brain interprets the ink on a page as opposed to something in three dimensions.
I think there is more to color blindness than we know.
My father was colorblind in the blue/green region. If he saw a blue something, for instance, he could not tell if it was blue or green and he was wrong half the time. Pretty much the same ratio as not seeing it at all. However, if he saw a blue and a green together, there was enough contrast to tell them apart consistently, even confusing aqua-blue-green colors on a page.
One of the people I used to work with could not see any difference in over half of a Pantone chart, but he was excellent at correcting for skin tones.
