There could be a lot of reason's for a different color in fescue. But in July and August, in this part of the world, fescue is under stress, it's not growing very much and a semi-dormant look is normal and is actually preferable. If your customer admire's someone else's grass it might just be the difference from irrigation or not, the seed, the soil, the fertility, the plant density, the amount of sunlight, the height of cut or he might be seeing a yard that is going to crash and burn in August because it's being pushed too hard right now. Don't fertilize fescue at all in the heat of summer, be very sparing with the water. Ironite granule's will get you a darker green color but only for 2-3 week's. Can you post picture's of both yard's?
Ironite vs. milorganite? You need ironite, even though milorganite is gentle, don't fertilize fescue in the heat, ever.