noun. A visual defect characterized by the inability to discern blue and yellow.

noun. A form of color blindness. The person's retina does not respond to the blue color, due to lack of "blue" cone opsins.

noun. rare form of dichromacy characterized by a lowered sensitivity to blue light resulting in an inability to distinguish blue and yellow

"I'm dubious about Wikipedia's tritanopia test -- I can barely make out part of the 5, and I have excellent color vision!"

deuteranopia  protanopia  

yellow-blue color blindness  yellow-blue dichromacy