I agree with EntrReality... to a point. I was a gymnast who constantly jammed my ankles on vault & floor landings so I taped for competitions quite frequently. I learned the difference between the plain white tape and using a more natural skin colored bandage towards the end of my career. I always taped with regular white tape and then covered the tape job with CoBan (beige veterinary/equine wrap). Worked like a charm.
However, for some athletes there is no choice. You can not find the neoprene very often and for many manufacturers the material of choice is the black or blue neoprene. Some athletes recovering from knee surgeries use the hard fiberglass kind of streamlined fiberglass hinged knee braces. Unless you get the super expensive custom model that actually casts the fiberglass individually, black is the only color that is affordable or maybe even covered by insurance.
There has to be some amount of reasoning to it.
I do know that USAG R & P for the Junior Olympic Program, does not deduct for supports, bandages or braces of any kind. It is true that the colors and bulkiness of these items can detract a judge from a perceived line, making a straight leg maybe seem not fully extended or tight, or an unpointed toe stand out more, etc. My recommendation for coaches/parents/gymnasts is to make whatever they use as least obtrusive as possible, but if it just can't be made to blend... don't worry about. I can't speak for other governing bodies of gymnastics around the nation, i.e. USAIGC, AAU, YMCA, High School Associations, etc.