Invert is a toggle. It is 1 or 0. Invert or not inverted.
Since 50% inverted does not exist (yes it does in your mind, that is why your mind is so beatyfull) there is no tweening possible.
You can ofcourse dissolve from not-inverted-state to inverted-state. But even that can not force the inbetween steps to make any sense. Then again, if no inbetween step can ever make sense, we can just feed something nice to the eye. In video i would probaly go for a dissolve that blends in a linear color space, or a soft circulair wipe.
This is an example of just dissolving from one state to the other.
https://www.dropbox.com/s/n7bpvdgvario9 ... .capx?dl=0
Hope at least you have something at the way how i use a timer.
Greetings.