Actually, genetics is super-complicated. It is possible that the green/red undertone gene is carried on the same chromosome pair with other sexual characteristics, which would make the finding consistent across races and ethnic groups. Red or green undertones are not affected by ethnicity, anyway. As you must have noticed, people of the same race and even the same family have very different skin tones.

Eliminating subjects who are unable to differentiate color does not void the study. Using subjects who couldn't tell the difference between the pictures they were seeing would skew the results, however. It's like this: A certain percent of the population is unable to run, for whatever reason. Eliminating these subjects form a study on running shoes would just make sense, right?