Because I asked on the forum of there is a way to give the mycelium some color in a biological way I also did some experiments. Thanks to the tip of @Elise I tried using natural fibres in the substrate and mycelium. I used some pieces of jute, cotton and wool. I randomly put some pieces in between the mixture to see what will happen after some time.
( sorry for the bad pictures , the light wasn't working along as well....)
Picture 1 : wool fibres
Picture 2 : jute
Picture 3 : cotton