I had the same problem with yellow tray icon and found out what was causing it.

I've been messing around with win services, setting 'wampapache' and 'wampmysqld' to automatic, but somehow, don't know how i turned 'wampmysqld' to disabled (similarities in words in my language). so i was starting wamp and getting yellow icon for "no reason".

i went back to services and set both to automati, waited for few secs, restarted wamp and voila, works like a charm.

eventualy i changed both services back to manual, starting them with wamp start.