Growing up, we were forbidden to wear nail polish at our studio. It was considered strictly taboo, as offensive as wearing a messy ponytail or a necklace to class.
I remember I once had an audition just 3 hours before prom. I had to paint my nails in the car afterwards on the way to the dance because I was afraid that if I'd worn polish to the audition I would get cut. (I ended up getting cut anyways, but I think that probably had more to do with my being so distracted by what I was gonna say to my date that I couldn't pick up the combinations. I probably should have just skipped that audition...)
One of the joys of taking open classes nowadays is being able to wear anything I want: a skirt, black tights, leg warmers, and even nail polish. It sounds silly, but I love dancing with nail polish. Bright red, sparkly silver, or neutral pink, I don't care; I love having color at the tips of my fingers. During plies, I watch a splash of color float through the air as my hands go from first to second position. My favorite is elonge. I love looking over my hand and seeing each fingertip perfectly placed in an elegant, classical position.
I understand why students shouldn't wear nail polish in class: It looks messy in a studio full of black leotards and pink tights, it undermines the discipline of the classroom and it's not something you should ever wear onstage. But at the same time, I know it helps me polish my port de bras (excuse the pun). When my nails are colored I think about my hands more than I usually do, focusing on how I can use them more gracefully without becoming affected. I think there's value in breaking this rule once in awhile, if only to remind yourself to leave no body part ignored. If you aren't aware of where you are placing each limb, muscle and finger nail, it might not be going to the place you'd like it to.