Lately I've been making a study of stills from classic films. It's a great way to learn about composition and colour. Here are a few I've done recently:
Colour Study. From Diamonds Are Forever (1971)
Compositional study. From O Brother, Where Art Thou? (2000)
Film studies have helped me with storytelling, as every shot is deliberately composed to help the mood or narrative. I hope to translate what I have learned into comics one day.