Here in the Seattle area I'd need shades of gray! Actually when we do have blue sky it's a beautiful blue and when it's a lovely blue with big white fluffy clouds it can be absolutely stunning. It took someone coming up from California to help me appreciate our sky here, she'd just sit and look out the window because, as she said, it was always changing.
If you'd prefer an afghan, why not? You could pick your sky colors and make them up in any pattern, why limit yourself to garter stitch? Take your inspiration from what they're doing but do your own thing.
I've wanted to try working a skyscape in Tunisian crochet to use as a wall hanging or pillow top or something. I could do that while you do your afghan! We could have a "The Sky's the Limit" thing going.
I should never overestimate my basic arithmetic skills.