The best type of paint for vinyl siding is high-grade acrylic latex paint. It has excellent bonding abilities and has enough flexibility to withstand the expansion and contraction vinyl siding repeatedly undergoes. Look for one that has a blend of urethane and acrylic resins.
Ideally, your new vinyl siding color will have the same shade or a lighter shade than the existing color because the siding was designed to absorb a certain level of heat. If you darken the siding color, it may absorb more heat than the siding was designed to tolerate. Buckling or other distortions could then occur.
Scour the siding thoroughly to have all mildew and dirt removed. You can do it by hand or with a pressure washer, but hand scouring will likely be more effective. Use an oxygen bleach solution if you have mildew, or a vinyl siding cleanser you can purchase at hardware stores. Rinse the siding thoroughly, and allow it to dry completely before painting.
Though you may think a sunny day would provide an ideal time to paint vinyl siding, you must take care not to paint when the siding is in direct sunlight. Instead, paint on an overcast day, which will provide the best conditions for the paint to dry and adhere.