Make sure you've consulted your paint supplier and you have the right paint product and color that will work for vinyl siding. Some colors don't work because their pigments attract too much sun and heat the vinyl beneath, causing the paint to fail. And it isn't a matter of "the lighter the better" - there are some dark colors that work better than lighter colors. Benjamin Moore, for example, has a collection of colors specifically for vinyl siding. Now, to answer your question: it's usually a good idea to paint when it's cool out. Paint, especially latex- / water-based paint, dries fast in direct sun and ideally you want to keep a wet edge as you paint your siding. Good luck!