I run uTorrent 3.1.2 in my Windows 7 PC. When I download one file, I see some purple colored lines under Files Tab > Pieces. Below is an image of what I am telling (little resized);

I think that the light green color indicates downloaded parts. But I have no idea about purple lines. The file is a streamable mp3 file.

The connection speed is very low, about 5KB/s down and 1KB/s up. The done file size is not progressing in a smooth way (usually changes in KB are visible), it stays as it is for sometime and then changes to a size (changes in MB), and again the same thing.

Questions:
Why does this happen?
What does the purple color mean?

Thank you.