Larger and wider shoes, plus keep your toenails very short. In the hiking and running communities, black toenails are common, so you learn to keep the nails as short as possible. Toe sleeves might help as well.

I had a terrible time with my teeth turning black. It turned out that I had too much iron. I stopped cooking all my .meals in cast iron pans

I searched the internet for information on Parkinson’s and nails and found very little.

Here’s a thread from the question section of this site. The last entry seems to be a possible solution.

https://www.myparkinsonsteam.com/questions/sear...

Not had any issues with toe or finger nails turning colors. However, I do have some issues with my toes and nails.
My toes are beginning to slightly curl under. My toe nails have all become thin and fell off at least once in the last couple of years.
My toe nails just get loose and fall off without pain or bleeding, etc.. it's difinitely strange. I've went to a podiatrist and he ran some blood test, etc.. but frankly was not any help.
They grow back, but they are very thin.
My finger nails have not fell off, but they are very thin and will break off if I don't keep them trimmed down.
Is it Parkinson’s, age, or just a weird quirk? I just consider it another one of my many little peccadilloes associated with age.

White paint? Sorry. I would see a podiatrist.