Thank you! It's digital. That was before I had SAI so I did all the work on Cthulhu in Photoshop. I made a base drawing of him using soft brushes, then I put my own photos of textures (stuff I had cooking on the stove, dirt on the ground, just anything can become useful) and set them on Muiltiply or something like that. I colored over and built it up in layers, experimenting with blending modes like Overlay, Color, and etc until it all looked how I wanted.
I revisited it recently and added 3D fractals in the background, which I also messed with different blending modes until they looked how I wanted, and then painted some blue here and there and set that to something like Color Burn? Don't remember.
I can tell you I find Overlay and Color Burn to be very useful and I often like Soft Light better than Color when I'm adding delicate layers of color. Make sure to merge layers like these down onto a solid (Normal) layer below it when you're ready to commit a change, or your blending modes may mess with each other when you try to flatten the image later. Don't be afraid of masks and Gradients either.
Lastly I find Luminosity to be pretty useless in Photoshop and like Color Burn or Overlay better, but I adore Luminosity in Paint Tool SAI.