Actually, my father is color-blind, so whenever I see Red and Green together I consider the option.
it doesn't affect him much, no problems driving or in daily life. he has a hard time telling flower colors apart and it affected how well he could play bejewled.
color blindess is rarer for woman, even if they pass the genes on to their sons. but simply knowledge of the condition means they have half the answer, and if they're smart enough (like MaoMao) then they get integrated back into the royal bloodline. so it checks for both bloodline (since they are taken from that region) and for intellegnce.
funny how Jinshi's mother only bowed down when the empress doweger came over, not for the previous emperor.
I'm not sure anyone can know who's the real father. we assume that Aoudo is the mother and the babies got switched, but we can't be sure it's really true, or who would be left to know about this. besides, digging this up doesn't benefit anyone.
Lakan being her father isn't that much of a secret, defintaly not important enough for them to hide it from the emperor. Lakan knows and told them himself, and maybe even told the empreor already (not sure how high up the food chain he actually is), maybe it was Gao-san who reported that to the empreor.