I . . . . . .was told that the regs required the test to be done with the prescription lenses worn
Can you be more specific on this ? Can you (or your AME, who told you) explain just WHICH regulations require that ? This is news to me.
Ishihara plates are best presented for interpretation in "
good daylight", and, if at all possible, with nothing else between the eyes and the plates. MOST (not quite all) glasses wearers should be able to identify the numbers without their glasses if the plates are held at the appropriate distance.
It is actually well known that certain types of tinted glasses/lenses can interfere with CV perception and/or testing. Not usually a big problem in practice, since CV critical situations are much less likely in bright daylight, which is when "
sunglasses" are most likely to be being used !