White buffalo (in a farm): Whithin this context, such mutation is neutral, because it doesn't affect the fitness of the animal.
Cancer: this mutation is harmful, since it could lead to the death of its carrier.
Disease-resistant potatoes: this mutation is helpful, because it increases the overall fitness of a it carrier.
White buffalo (in a the wild): white buffaloes in the wild could be more conspicous for their predators, therefore, this mutation is harmful.