In my experience most colorisation looks very smudgy, does not "colour in between the lines", and generally seems to make things look sepia with low contrast - however I'd like to see this problem investigated more to see how good it can get - perhaps we could end up doing similar things for astral objects?
There's also a bot on Reddit that colorizes: http://www.whatimade.today/our-frst-reddit-bot-coloring-b-2/