what is the meaning of red downloads.. some are red some are black?

What client are you using?

In BearShare, the red bar means that host is already busy uploading to as many people as they can. A red file name means that host is behind a firewall, normally you can still download from them though.

Questions like this are client specific so you should post them there not in the general area.

I think i know what u mean, and the reason is those are for some reason or another unreachable peers. All u have to do is go to ur preferences tab, then click on the tap that says " Downloads "
just check the " ignore unreachable peers " should solve the problem