to make a comparison to something all readers can understand
Explanation:
The user mentions the color red in order to make a comparison to something all readers can understand. Everyone has been taught colors from an early age. The reader is using this expected general understanding of the color red to compare to the logic and principles of geometry. Stating that what we are taught as a principle is what it is because we are taught that way and no other reason. The author uses the color red because in the same way as lines in geometry, we have been taught what red is and what it looks like.
