While downloading files from utorrent you might have noticed in pieces tab there is a column for availability. It shows different colours viz. dark blue, pale blue and red. So what does these colors mean and how they illustrate the availability of torrent.
The color red means the piece is not available amongst the peers you're connected to. (this means availability is less than 1 and you may not be able to complete the torrent)
Pale blue means that the piece is present amongst the peers you're connected to, but rare.
The darker the blue, the more common the piece is in the swarm.
From Mininova .Fan's Guide
Further useful Articles
DHT in utorrent
Find more active trackers for torrentz
Free trackers to increase torrent speed
Good Share Ratio in torrents
Ways to increase torrent download speed