I use blue for guys I will consider my “primary targets” - in other words, guys I will be willing to spend substantial AP/$ on.
Green are backup or project players I may try to secure with AP - these are typically low starting, high potential players, and if I do invest in them, it will be calculating that potential human rivals won’t want to invest much, either.
Yellow is for late signing “plan b” or “plan c” types, guys like green, but with a late signing preference. I might try to get in position on a player or two in this category, especially if I’m worried about a possible early entry.
Red is for high level guys I want to watch and put early points on, and may move to blue, if I like how the early battle shapes up.
Your mileage may vary.