Michael,
I'm with John Dozier in some ways. A 18% neutral gray is easiest on the eyes so that you can focus on the subject. However, if you must have color (gray is not a color, IMO), pick an intensity that would reproduce around the 18% gray if shot in B&W. The specific color then just becomes a matter of tastes. BTW, the white/lightest background makes it difficult for me to focus on the subject as well as a neutral intensity background. The black makes the subject flare a bit, to me.

Solid backgrounds may be a bit plain, but there is no distraction for your eyes, if the pic is for technical identification of features. If artistic presentation is the goal, by all means, go with Thorney's suggestion of natural backgrounds.