I know what you mean (I think). For me, changing my voice often triggers feelings that I am losing something. People loved my male voice. With it, I could be engaging and persuasive. I could use it to express deep feelings and to invite trust. I've lost that - for now at least. Who does that make me?
In such moments, I try shift from thinking about what I am losing towards thinking about what I am letting go of. Letting go of something implies freedom. You are now free to explore new experiences and feelings. In your case, you can explore what nail polish means to you. Is it something you do to meet social expectations and to fit in? Do you experience it as a chance for self-expression? Or maybe you find it a time for quiet self-reflection and self-acknowledgement (while you put it on)?
In my case, I am definitely letting go of one of my most powerful instruments. But I also have an opportunity to develop a whole new emotional vocabulary and to access relationships in new ways.
In any event, the wonderful thing about letting go of something (as opposed to losing it), is that you can usually reclaim it if you find that it isn't something you want to let go of. And you will hold on to it with a deeper appreciation of why it is important to you.
Jump in the river and explore the currents!