Thought I had only 1 idea for 7? Actually, I have a few others, but there's not much to thm since I can't really think of any other colors. The yellow 7 is based on the "7 eats 9" reference I've kept for quite a long time. The backstory wa Himitsu dayo. The Blue 7 is based on some title headings for when Episode 5 was split into 11 smaller short-sized pieces.
c2a I also... have a really lazy 9 prediction in the works.