It's not a cooldown... is the time remaining on the debuff you placed on the target.
There are four colors used on the action bars to show timers...
If you place an aura on the target and that aura is on cooldown, then the button counter is shown in purple and tells you how much time is remaining on the debuff (not the cooldown!)
When the cooldown expires and the aura is still active on the target, the time remaining on the aura is shown in blue meaning the spell is still active, but you can recast it if you like.
When the associated spell is *not* active on the target but the action is still on cooldown, the cooldown counter is shown in yellow until there's less than ten seconds left before it expires after which the cooldown counter turns red to alert you that you'll be able to recast it momentarily.
- purple is time remaining on an aura that is also on cooldown
- blue is time remaining on an aura that is not on cooldown
- yellow is time remaining on a cooldown over 10 seconds with no applied aura
- red is time remaining on cooldown under ten seconds with no applied aura