Just came across an issue where I started WAMP, but the icon went orange instead of green.  Hovering over the WAMP icon in the System Tray, I saw a message like this:

local server – 1 of 2 services running

Turns out that I had Skype running and it looks like the port that Skype uses (80) collides WAMP.  Closing down Skype and restarting WAMP did the trick.

Also found a reference on the subject.