When toning a canvas, the trick is to pick colors that make your job easier.
For a sunny-day sky painting, I would tone the canvas with a warm color, to add warmth to the final painting. My go-to is probably raw sienna, a simple and easy tone. However, I'd choose a color that somehow unifies sky and land, so if I have orange elements on the landscape under the sky, I'd pick an orange(ish) ground color.
It's also a good idea to vary the toning color in different areas of the canvas to support the variations in the final painting. So where the clouds are going to be placed a lighter version of the color works well. If you are using oils, avoid mixing white into the ground colors because it dries slowly and it would end up creating a chalky effect on the next layers of paint. Light it up using a lighter/brighter yellow instead.
Another option for toning would be to cover the canvas with a bright version of the sky color, maybe teal or cerulean blue. In this case, when you go to paint the sky, you'll find that your job is already halfway done in several areas.