Just don't put them in the scene is the usual answer. MacBeth color checker is a good way to check your color code. Now is the time to switch from RGB to spectrum if you plan to use the color checker at the end of the quarter. Picking something on the front isn't picking from exactly half the sphere. Is a graphics gem on this.
Trouble is light scatters off the medium and indirectly lights stuff in zillions of ways.
Multiple scattering sol'n paper has been submitted to siggraph this year. Maybe a lecture at the end if it gets in. He measures the closest distance that a ray comes to the object. Gives a shadow that's about the right shape, about the right blurriness. For small objects, makes big unphysical penumbrae.