Yes, you can but it's not advisable. Shingles lose their granules over time. What you can easily wind up with is a splotchy looking roof. Keep them painted every few years though and you will be okay, Another thing, shingles are made so that there are variations in coloring. With a painted roof, there will be no variations. It will be all one color.

If the hail is big enough, then it won't matter what you use, the material will still get destroyed. There are some shingles that can take quite a hit and remain in good shape. There are SBS Modified asphalt shingles made by Malarkey Roofing. These shingles have a class iv hail rating which is the highest hail rating available. You can also look into metal, but be careful. We've all seen the hoods of cars after quarter-sized hail storms.

Don't go lower than 2:12 which means two inches of vertical rise for every twelve inches of horizontal run. Anything between 2:12 and 4:12, you should consider installing two layers of #15 felt underlayment, reducing the exposure of the shingle - the exposure being the part of the shingle that remains visible after installation, or both. In lieu of both of these, you may want to install a self-adhering ice and water underlayment over the entire deck surface.

Felt paper is a vapor barrier plus keeps the oils in the shingles from leeching out and into the wood substrate ( i.e. plywood or OSB roof deck), so #15, or 30 will work for any shingle roof. The type I recommend depends on roof shape. For slopes less than 4/12, I recommend 2 layers of #15. Helps reduce the possibility of leaks. The felt overlaps in " shingle fashion". Slopes 4/12 to 6/12, I recommend one layer of #15. Slopes greater than 6/12 I recommend one layer of #30. The steeper the slope, the easier it is to slip and tear the felt. #30 is heavier so on steeper slopes, you don't slip and tear it as easily.