I had that sort of problem once. I think it was rather soon after conversion, and I didn’t ask anyone about it, just decided (for some time) that it’s probably vanity and gave it up. But it turned out some time later, that seeing as I still knew little about faith and virtue and so on, I stuck too much with some externals. (Not that I ignored internal life) And it gave rise to scruples. Somebody here wrote about it, and I think it is very possible in your case. And it maybe quite dangerous - I read about it in a number of places, including some of the works of Saint Alphonsus di Liguori. With the help from my confessor and some work on my part, I gradually paid less attention to strange details such as that and gradually scruples went away, it wasn’t easy though.
If - as sb else wrote - it is vanity, that would mean you ascribe to your looks and what kind of impression you make on others - too much attention, then, even dying your hair maybe a venial sin, though it’s be rather a weakness or imperfection, in my view. But when it’s just part of how you take care of yourself (you know, clean, fragrant, decent looking etc) and you live in the world, then I don’t see a problem at all.
One more thing related to the subject - I noticed that sometimes looking well (but not extravagant or shocking, indecent etc) is part of … evangelization. There is a line of course. You may be a shabby looking saint and do a better job at drawing people to God every time. But in some cases - with atheists, women especially, mostly young and rebellious - they look at you and think - well these Catholics look like thrash, I don’t want that. In other words, it may repell them that you don’t take care of yourself at all. It’s about not going to extremes.