Concrete walls with PSI higher than 4200 may not require sealing. High pressure sprayed concrete (gunite, shotcrete, mortar) often times will test out higher than anticipated based upon the mix. I think this is so because of the low water-to-cement ratio and the compaction resulting from the high air pressure.
Just to be safe, even with a high PSI, I would seal the roof. Various products for sealing. Some are penetrating and some just seal the surface. Look at Foundation Armor S2000 for penetrating the concrete. If shrinkage cracks develop, then you may need a surface product that has good elongation properties.
The roof should cost less if the concrete is cast-in-place instead of sprayed. A steep roof will usually require that the concrete be sprayed. Including color in the mix will add to the cost but will probably be much less over the years than painting. I guess you are aware that gray cement will limit the range of colors. White cement will offer brighter colors but will really add to the cost.
Quikrete makes a bag mix with gray cement. I think it is made in Birmingham, AL. There may be a plant near you that could reduce freight charges.
I think that adding color to the mix will lend itself more so to pressure washing than painted concrete. Even if the concrete is not painted, concrete finish can be damaged if the pressure washer uses too much pressure.
You may want to get a quote for pre-stressed roof panels. If so, then be sure to add cost for the topping slab and insulation. There must be work space around the building for precast to be considered since a large crane may be required.
E-mail me if you want a copy of the design for Permanent Anchors/Temporary Bucks for SCIP walls.