We live in Los Angeles and have a flat roof that our hot and cold water copper pipes run across and are all exposed. The cold tap water, in the spring, summer, and fall is so hot during the day. Currently there is only black foam insulation on it. I am thinking there has to be something that will reflect the heat off of the cold water copper pipes and insulate it too.
I would try white, silver, or foil pipe tape. Even for high quality tape, it is cheap. You can simply wrap it over the existing black foam.
Caveats: I live in a net heating climate, not a cooling one. I have no experience with flat roofs, nor living in desert climates.
Out of curiosity, how much of the time does your air conditioner run. If the pipes are getting that hot, you may not have much insulation in the roof. Could be a big win for you to put down 2-4 inches of foam board insulation on the roof and reseal.
You want to do two things: You want to cool off the roof, and you want to keep what heat that is there from heating the cold water line.
Try this as a thought experiment: Buy some 2 foot wide foam panels. Ask for stuff used for insulating flat roofs. You don't want one that turns into goo when exposed to roofing tar.
One of the panels is 1" thick. Rip in into 4" wide pieces. Put a piece on either side of the pipe. Cut another panel into 8" wide pieces. Lay this on top of the pipe and the 4" pieces. Put bricks, stones, or kids needing a time-out to hold the foam down.
Do this for most of the length of the exposed pipe.
See if this makes a signficiant difference. Between the foam being light coloured (white, pink or blue) and it insulating the pipe from the roof temps in general, I suspect that it will go from being dangerously hot to just warm.
Ok. If it works, you need to make this more permanent.
Take as strip of gravel off the roof so that the foam fits the roof better. Doesn't need to be perfect. Don't cut through the roofing paper or membrane under the gravel.
Apply roofing tar to the strip, and lay it down. Tar should be thick enough to make a reasonably tight seal to the gravel-less roof.
Repeat for the cover layer.
Get a paint compatible with the foam, and paint the foam white or a light colour. Put at least two coats on it. With the second coat, you can sprinkle sand on it to help protect the paint.
You may find it worth while to paint the entire roof. If it's a gravel coated roof, you may be able to get away with cheap white wash. This won't last very long -- probably need to redo it every 2 years. But whitewash is basically calcium carbonate and a binder, and is sold in dry form in 50 pound sacks. Should be available at most farm supply stores. Anyway it would be an inexpensive experiment to see if it cools the house or allows your air condition to run less.
You can just go to your favorite home improvement store and ask to see exterior pipe insulation. it is a foam sleeve with a space inside the size of your pipe, split down the middle to it can go around pipes that are already installed, then you seal it over the split with a type of tape. The outdoor versions then have an extra external covering to protect it from UV damage.
If it were me, I would probably also construct a box or cover over it to protect it from damage and to keep the sun off of it, it will last even longer. UV breaks everything down, some things just slower than others.
There are lots of different types of pipe insulation.
You also can get something that industrial quality like 4 inch thick mineral wool pipe insulation and cover it with white mastic which would give you a R10 or better plus the white mastic would reflect heat and the mineral wool is fire resistant.