Somehow Dwayne manages to reach his late teens without having a single eye test that would have picked up his colour blindness straight away.
This troper didn't get a formal color blindness test until she was 17. It is an optometrist's job to test for color blindness, especially for men, but it's usually looked over for everything else.
What's more confusing is that Dwayne's parents seem to at least be aware of the possibility (among other hints, Richard starts gesturing wildly in the rearview mirror when Olive takes out the color blindness test). Why did they not have him tested or, at the very least, tell him he might be color blind?
Richard was gesturing at the side mirror to the other cars honking at them, not at Olive.