I imagine depending on which laws of physics you want to retain and which you want to drop, there are some pretty deep and counterintuitive results. Since that's too hard let's do something simple.
Objects near a large mass (black hole) seem to have their time slowed when viewed very far from the large mass. The same sucking action of gravity causes light traveling away from the large mass to lose energy and become redshifted.
If your slow-time bubbles use a similar principle (a localised area of high gravity pulling equally in all directions) the answer is they look redder depending on how powerful they are. Super powerful bubbles have the frequency lowered so much they become invisible as the reflected light is turned into radio waves. So those bubbles appear completely black.
There is no such thing as a reverse black hole so this has no physical basis -- but it makes narrative sense the fast-time bubbles have their stuff look bluer depending on their power. Again really powerful bubbles have their light turned into radiation which is invisible. Again the bubbles appear black.
Likewise, if you're in a slow bubble stuff outside appears bluer and if the bubble is really powerful the outside world is just a massive black void surrounding the bubble.
It's also a good narrative device if the slow bubble is completely black. If there's a fight going on inside we can't see what's going on and we can't help without hopping in yourself and becoming trapped!
Calculating the redshift would depend on how the gravity in these bubbles. I suggest the gravity is very strong but pulls evenly in all directions and cancels itself out. As though you were at the hollow centre of an immensely thick and heavy dyson sphere: you are weightless and experience slow time.
The normal formula for gravitational dilation assumes light traveling away from a large mass with the normal sloped gravity well so cannot be used here. It's probably easier to estimate how powerful the bubble can get before it becomes black. Any takers!