"Fashion" does not exist the same way as before. It used to be that the TV and magazines would be fairly synchronised in what was going on, so fashion was fairly uniform.
Nowadays, however, there are very many different fashions all happening at the same time... and across many different types of media. The problem here is that if I do a Google search for "White toenails", i will likely be targetted with adverts and stories about white toenails. From this 'personalised' experience I could be convinced that white toenails are the highest priority at the moment. Another person however might Google "Lime green toenails" and they might experience 'personalised' adverts that convince them that lime green is the latest fashion. Multiply this by 100 colours, and suddenly everyone becomes a fashion expert - each alerted to a different colour.
So, I say that there is no such thing as a fashionable colour for toenails (or anything else for that matter). The only thing truly fashionable at the moment is diversity. Also fashionable is the false illusion that fashion exists - because that's what the manufacturers and sellers want you to believe, so you spend money on their 'fahionable' product.
Consequently, just be confident to be yourself irrespective of your perceptions of latest fashion. White, pink, sparkly, natural, neon, metallic, luminous, rainbow, pearlescent, shiny, gloss, textured, smooth, long, short, pointed, blunt, etc... (and the list is infinite) are enough to make you realise that the fashion sellers are just manipulating people to buy their product. It used to be uniform across the nation (e,g. most men wearing bowler hats in the 19...s), but now they sell us every colour and personally dupe us into thinking its fashionable. Ignore fashion. Be yourself and be strong.