There are a lot of slabbed "bright" coins that must have been dipped, so that may be to their whim. A lot of those Gothics have a purple tone that makes me think they may have been stored with some kind of felt or other similar material & then they have some of the other colours. You should see some of the PCGS posts on colour progression (I don't remember crystallography to well from chemistry, but looks like they took some basic truths and ran with it & IMO their opinions a load of BS). A coin retoned 50 years ago would I think not be so obvious and would likely generally escape detection. However, I have seen some coins that appear suspicious for old cleaning and not retoned, I think just through experience. I still think based on picture presentation that the OP coin is likely legit though, and have seen a few.