The new flagging summary (click on your number of helpful flags in your profile) is cool, however some posts have a gray background, some don't. What does it mean?
What I've been able to work out:
There doesn't seem to be an indication of whether the flag was marked as valid or not.
A flag could be dismissed as valid, but no action taken. Equally it could be dismissed as invalid while there was still an action taken. It will depend on the moderator reviewing that flag.
Certain flags are invalidated if they are of sufficient age.
Aging rules are here: When are moderator flags automatically dismissed as "aged away" by the system?
When a flag is aged like this, you see "aged away" for the flag reason...