I've been doing some more practice on mixing colours using Cathy Johnson's book Creating Nature's Detail in Watercolour, which has some great examples of doing interesting sky effects, and Nina Leland's amazing book Exploring Color, which is probably the best book on learning about colour theory and mixing colours that I have come across so far.
A quick go at one of the examples in Cathy Johnson's book. (6 x 10 cm)
The sun breaking though clouds and fog in the early morning (using Cathy Johnson's book).
The sun starting to rise over foggy mountains. Nina Leland shows how to mix various grey's using opposing colours, and while it is a bit more effort, I have to admit that the greys are more interesting than just Davy's Grey from the pan.