This is the fun part! Usually, when I start thinking I have something solid I can work with, I drop in a picture or an old painting of mine to get some colours and textures. You don’t need anything that looks like your subject – the goal here is to get some noise going, and produce some happy accidents. I usually set the image on top to Multiply, Overlay or Soft Light mode, but be sure to check all the layer modes – you could get some nice surprises and results. After this, I just sample colours from the result, because the colours mix with the values.