I am really not a huge fan of dye. I just don't think it makes that much difference.
Now, if you have a black and white one, and the white is at all of the wrong places...that is a different story. Say you have one with a white flank, white on their front legs, or maybe white on their tail head, it can really change the way that the calf looks. It can make them look short legged, shallow flanked, or funky in their top.
My advice is to dye the areas that detract from the quality of the calf, and leave the rest. This will keep people from thinking you altered the color of the animal in the event that you feel some people might complain about it. Every show has different rules....I am not saying to break the rules. You can decide on that on your own.