Why is the download icon (rotating circle) sometime all black and sometimes black and red?

Using iTunes on my MacBook Pro, when I download items there is a circle on the upper right of the Songs view of my library. The circle has a mark within it that appears to be an exclamation mark. Normally, the whole icon is black. At other times, the mark inside the circle is red and the circle itself remains black.

I wonder what causes the difference and what it means.