Orange blocks in the pieces tab

[bhamwildcat]

I am using the latest stable (not beta) BitTorrent. I have Win7 64-bit. Sometimes I'll sit and watch the pieces tab, and watch each block be downloaded. However, sometimes I'll have blocks and sometimes even entire pieces turn orange, and sit still for a while, like this:

Bit Torrent Screenshot

I know purple is downloading, green is downloaded, and dark purple is downloading in Endgame mode.

What does the orange mean?

Should I worry about it?

How do I fix it?

P.S. The manual in help (F1) does not seem to be updated to the current colors used in the "Pieces" tab.