It’s impossible to tell without seeing your code or your node structure.
My only guess is that maybe your color rect is behind something, or it doesn’t span the entire screen.
Did you create a RESET animation for the AnimationPlayer? If not, that’s likely your problem. It would help if you could post a screenshot of your screen, including the scene tree on the left and the animation on the bottom.
Thanks for all the replies.
‘Bug’ fixed by colouring the ColorRect black with full transparency by default. The animation then changes the rect from transparent thru full black.