If this was my Regina bag, I would wash and condition it to see how much of the colour could be restored. I doubt it will help much, but it is still a good start. If I wasn't satisfied with the result I would dye it to the original colour which I believe is lime green in your case.
Here is an example of a non-faded lime green bag which I borrowed from the internet:
So pretty!
View attachment 5598947
I dyed this vintage Small Shopper 9993 with Leather Touch-up Dye because it was covered in spots. I had sent the original hangtag to the company so they could mix the perfect match for me at an additional cost. Although it was an expensive investment the bag looks fabulous now.

The dye lasts for 5 years in the tube and I have 5 aqua bags to use it on if need be.
View attachment 5598939
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www.leathertouchupdye.com
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Good luck! Let us know what you decide!