James Gurney, on his blog, had a post a while back about the sky’s dual gradations.
Everybody knows that the sky is dark blue at the top and gets lighter and lighter down toward the horizon. But it had never occurred to me that there’s another gradient from the sun. The sky is lighter blue close to the sun, and darker further away. I don’t know why this never actually occurred to me before.
Anyway, I had the idea that I could duplicate this by two layers with two different gradients on top of each other. One gradient shades light to dark, for the horizon-to-zenith gradient. The other goes sideways, shading from warm turquoise to darker blue. It took some tweaking of the layer blending settings, but I got a look I finally liked.
I put some clouds and ground and dinosaurs on there just so the eye has somewhere else to go. Staring at just gradients gets tiring.
In case you can’t see the gradients, here’s some helpful arrows:
Gurney does it better than me, and with actual paint, too. See his post and example pics here.