Your best bet is to take someone with you and pick from the vast array of colors they can mix. They will not have pre-mixed in the colors you will want. Normally sky can be from a light blue to almost white, but can range from a purple to reds and oranges. Like Kevin says, the choice depends on the lighting you use and even the effect you're trying to achieve. The color of dirt will vary as well dependent on the area your trying to model.
When I did my backdrop, I started with a light blue and kept mixing white into it as I got closer to the horizon. I can't say I did a great job, and will probably paint over it someday, so I'm not the best one to give you advice on that aspect.