Turns out I have
another mental "disorder" in addition to being INTJ. My mental numbers have colors.
According this wiki article, I have associative Grapheme-color synesthesia. Mine are based around the primary color mixes. 2 is red, 3 is yellow, and since 2+3 =5 , 5 is orange. 4 is blue, so 3+4=7 and 7 is green. However 9 (4+5) is purple.. for no readily apparent reason. 8 is black. Other numbers don't have a color. I don't know if it is based on some toy, or magnetic letters, or a book, but this all got stuck in my brain.
I asked at the table what color 5 was, and my daughter agreed with me, its orange. But she couldn't explain why either.
Anyway, does anyone else here have this "disorder" ? From the examples, it almost looks like it can become a super-power.