I'm in the southeast (humid area). I don't know anyone who has painted their roof, as others have said roof is a harsh climate for sun, rain, etc. It seems paint (especially white) paint would not wear well for this application, and maybe reduce lifespan of the shingles.
If you really want to do it, find a least a couple people who have done it in your climate (with same roofing materials), look at the results, ask questions. Otherwise you may be making a perfectly fine roof look like crap with no cost savings.
Once your roof does need to be replaced, there are plenty of light colored asphalt roof shingles you can use, and oftentimes they have a rebate associated with them.