I have had this happen to me a lot and what usually fixes it for me is to close the game and reopen it, also you can let it finished with the playback speed set at 1x and see if it goes from gray to black.
If that does not work try going to another replay and see if it does the same thing, I have found sometimes that fixes it. I am not sure what else to try other than that. Usually this thing fixes itself though.