colour theory:
Color—the theory and practice of painting [PDF]
drawing:
Summer Drawing Starter Pack [tutorial playlist]
https://line-of-action.com/practice-tools/figure-drawing
painting:
How to Match Any Color with Oil Paint
How to Paint Flesh Colours Using the Zorn Palette
software:
https://www.gimp.org/
https://krita.org/
https://inkscape.org/
https://www.darktable.org/
book:
The War of Art [audio]