I've heard you can buy cultures of different spalting fungi for different colors and spalt the wood that way. It needs to be in the right conditions, warm, humid and shaded/dark. I'm learning more about spalting and the different fungi myself to try and eventually spalt entire logs using cultures. Look up a woman named Sara "Seri" Robinson. She has pretty much dedicated her life to understanding spalting and what fungi produce which effects and colors. She has a book published in 2021, "Spalting 101." I'm waiting for it to be delivered in a day or two.