Yeap, photoshop is the way to go. If you don't have it there are lots of other freeware alternatives to do same thing.
Instead of painting it white I suggest go and buy some white adhesive film and apply it on the roof. Same stuff that is used for vehicle decoration/lettering/advertising.
I guess you want to do this in preparation for a trip or a temporary staying in a desert area. This way you can revert to the original roof color in no time. Just make sure you buy a film with a removable adhesive.
Another option would be to use silver instead of white. Should look a little bit better and it might reflect more heat, it has tiny metal particles in it that reflect light.
Another thought is that if you'll have a roof rack on the disco you can do something on the rack itself if you won't have it full of stuff.
On the other hand I vaguely remember of someone doing a test painting the roof with some white paint you normally use when redecorating your home (a cheap stuff that could be easily removed going through a car wash) and he got only a 2 or 3 degrees lower temp. when exposed to sun in a 30 deg. C (or more, can't remember well, a very hot day anyway), from 48 to 45 deg C inside, stand still, windows closed.
I didn't say that to discourage you. Just to tell you not to expect much improvement.