He was too absorbed in solving this problem, and when he did he became truly excited to try it out. Missing such a trivial point was expected, and yet this little detail wouldn't cause any problem at all.
So when he started training, two weak seeds of these elements started to form into his spirit. As he didn't absorb a monster spirit directly, he didn't add any more tails to his two. And so these two elements overlapped over the other three, coating the two tails with two external layers, one red and one green.
William was already absorbed in training, sitting cross legged, regulating his breaths, and muttering the incantations once in a while. He kept moving his spirit power inside his body according to the diagram he created, while his spirit kept absorbing light and lightning spirit elements from the world around, fire and wind elements from the cores surrounding him.