I've had good success using Krud Kutter and a microfiber cloth. I've always attempted to save the numbering so I was aware of how hard I was pressing and whether or not the ink was starting to run. I suppose if you didn't care about keeping the original numbering you could clean thoroughly and then paint new. Might stand out in a bad way depending on the rest of the cab. Some yellowing I find is removable, such as nicotine staining. Others not so much.

Here is a good example of how too white doesn't look right. I printed off one dummy reel in pure white and the other in a cream color. The cream matched the original reels near perfectly. In fact many paint schemes (cabinet/playfield/apron/exc.) were not perfectly white but more of a silver/cream color hue.

Pinside_forum_6215887_2367396 (resized).jpg

An ultrasonic cleaner and a little Simple Green work well. I've had this guy for about a year and a half and love it:

https://www.harborfreight.com/25-liter-ultrasonic-cleaner-63256.html

See if a magic eraser does anything. Maybe it needs the 'retrobrite' treatment?

I second this. Might need to soak in peroxide and leave in the sun for a while.

what are you using for cleaner in your Ultrasonic?

I use Men Green in my sonic cleaner. I would let them soak a bit too.

After that long in an ultrasonic cleaner, you're probably as good as you're going to get. Peroxide and sun will usually lighten them up but will probably be blotchy.
If you are making all new labels anyway, there's no need to go any further. The white labels will cover all of the reel that is visible.

Looks Great!

Nothing wrong with that result. You will probably find that the notches will end up having some paint delamination because of the wear as it steps. My guess is it won't be noticeable from the score windows. Huge improvement.