Mine is pretty much a clay color (think vinyl siding) but probably a little darker. The back wall is actually a dark green stain instead of paint. All trim is white. Honestly, the walls might as well be black in terms of light reflection. After all these years the walls are probably 75% covered and that number goes up every year. The ceiling, however, is white and that reflects the light (from the lights that just so happen to be mounted to the ceiling - imagine that!) quite well.
I just can't do white on a garage wall - it shows everything. If your walls are OSB like mine and you hang things in random places then move them later - the holes aren't nearly as noticeable with a darker color.
I have no scientific evidence to back this up but believe me anyway - the color 'clay' was invented because 'clay' is a major component in 'dirt' so any surface with a 'clay' color doesn't show 'dirt'.