color correction power windows/ mask - key framing/ control in FCP

i'm trying to fade in and and out power windows (aka color masks in FCP) on a particular shot. all i've been able to figure out is to key frame the window (color mask) so it can move around the frame tracking with a face or whatever the case may be. but if i want to physically turn the entire window (color mask) on midway through the shot and back off again before the shot is over (with fade in / out - so it is imperceptible) - i haven't been able to figure out how. i'm accustomed to using resolve and working in big color houses where we have complete control over any effect for that matter.

can anyone shed light on how (or IF) this can be done?

thanks in advance!