Although, the dog could tell the difference between the swords. Dogs are blind to red which means pure red things are like an absence of any colour, so red would appear black. Dogs can see blue and yellow which combine to form green which to them would be all of the colours, and so it would appear white. I'm sure an anthropomorphic dog would understand that when humans say "green" they mean "white" and when humans say "red" they mean "black".