Drawing with pen and ink isn't sometimes enough to express what you want. Occasionally I add watercolor to my drawings. For long I've had troubles painting the sky, until someone taught me how to: wet the paper, use a light blue and a darker one and then 'paint' the sky with a sponge. Being not afraid to mix the two colors on your paper. For me this gave fantastic results, I must say. And it's also a quick method to paint an expressive sky.