On the standard board. I remember seeing an explanation, or think I did, but there's nothing in the Help-FAQs section.

Specifically, on the posts I see at least the following highlight colors:

Baby Blue
Air Force blue (usually a quote within a Baby Blue post)
Apple Green ("staff" posts, I think?)

OK, I made up the color names, just what they looked like to me.

If I don't know what they signify, they just confuse.

Green are posts made by certain members of staff.

A post will turn blue whenever they are given a certain number of blue stars. (usually 3-5 stars),