What does the color of a level indicate?

[BunchofCrooks]

Once completed, on the main campaign map screen, the levels seem to have one of three colors; red, blue with a star, or black. What do these colors indicate?

[Rarst]

Red are free levels, blue are levels that require paid subscription.

Not sure about black, it had appeared like yesterday.

[nick]

Black levels unlock a new hero for purchase in the hero shop.

[dwhittaker]

Also it appears we get a red flag if complete and a grey flag if started but not complete.