What does this glyph (red) mean? Also, what usage was there for it and the other glyph that looks like a colon (blue)?
You're looking at a scribal abbreviation for "-rum". That is, the word is actually philosophorum, but with the last three letters replaced with ꝶ. Here is a screenshot of the enlarged character from the graphemica page:
The colon is probably a punctus elevatus which is sometimes written without its tail, i.e. like a colon. It's basically like our modern comma.