For the Sun light :
- full sun : 255,255,220
- dawn/dusk, linear fade to : 255, 60, 60
- then to : 0, 0, 0
During 2, spawn a second light source, like the moon, with a blueish tint, like 20, 60, 180, placed 180° opposite to the sun.
Don’t forget to rotate both lights around your scene.
Some sky gradients may be nice too, from blue to orange to black.
For something more realistic, for both the sun and sky, search for Mie and Rayleigh scattering functions, like these :
http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter16.html
http://forum.celestialmatters.org/viewtopic.php?t=116&postdays=0&postorder=asc&start=60