The minimum distance from the back wall doesn't relate to "coloring" the sound but rather giving the rear ports room to "breath". A few inches are enough.
The most important factor in placing a speaker so that boundary effects with the nearest room surfaces don't create uneven response is to have the center of the woofer cones at different distances from these surfaces(floor and two walls). This is to minimize the boundary effects piling up at the same frequencies, causing a big bump and a deep dip.
