I've been thinking about this since I posted and came back to add that I think the issue is that many people associate "looking good" or "taking care of oneself" with adding colour to one's body. So - this idea that if you have grey hair you should wear bright lipstick, or the implication in the post I quoted before that self care includes colourful nails.
I do care for my appearance - I am clean, reasonably well groomed, I do look after my skin even though I don't use lots of expensive potions and lotions. I may not spend a lot on my clothes, but I think about what styles flatter my figure and what colours suit my complexion (and hair).
There is a really deep-rooted expectation that women must physically add colour to their bodies - hair dye, nail polish, make-up. Women are expected to be ornamental and decorative in a way that men simply are not. If you want to test this - try looking at photos of men - in a professional or leisure setting, and imagine them with make-up and colourful nails.
Some women are prepared, or happy, to conform to those expectations, and some of us prefer to look good in a less socially expected ornamental and artificially coloured way!
For what it's worth, I stopped dying my hair when I moved to a house where the bath and shower layout made it more of a hassle. My only regrets are that I didn't do it sooner, and that I didn't just go for a buzz cut at the start instead of going through the grotty growing out process.