Think balck and white, middle grey is the perceived mid point in brightness between pure black and pure while, it is not a color as much as light intensity. You camera tries expose so that tone is mid-way in the capture range of the sensor.
If you are thinking of a bounce surface for the flash, look at the direction you would want like coming from to illuminate your scene and trace backwards where you would have to put the flash, pointed at the reflecting surface so that when it bounced and diffused the light would be coming from the angle you want. Find as neutral a surface color as possible, not shiny preferably for better diffusion/scattering, and you will be amazed how much a small amount of controlled light can help and hardly be noticed by the subjects. I scout a venue before an event shoot and find whatever bounce surfaces I can direct remote Slave speed lights towards that would help. I try to find a vantage point above the crowds to tape or bungee the speed lights so no one will be messing with my SB900's Very little additional light is needed to be able to shoot at 1/200 f/2.8 to f/4 on DX and f/4 too 5.6 on FX at a reasonable ISO, in placed tht otherwise would have required 2.8, 1/60 and 12k ISO. Stan St Petersburg Russia