The teacher is in the flower store. He smells a red rose and says "roses are red", and then points to a spray of blue flowers and says "violets are blue". These flowers appear to be Delphinium, or something tall, but are definitely not violets. When the owner of the shop comments next, she doesn't correct him, but rather discusses different colors that violets can be.
The type of colour blindness Ryan Tanner is portrayed as having isn't correctable by special lenses or glasses.