Neutral colors--an oxymoron
The rule in color theory is that neutral gray or black is the result of mixing two complementary (opposite) colors on the color wheel. It works in theory, but not in paint. Very few colors can be described as exact opposites and almost none are true hues in the color scheme sense. Sometimes you get black or gray when you mix, but often you get brown or umber. The paint colors are actually "near complements" and make a mixture with a bias toward one of the two colors in the mixture. This color combination makes a more vibrant mixture.
One thing to remember about mixing neutrals is to use only the colors that are already in your painting. Plan your colors before you paint to make sure you can get your full range of values, color and neutrals. Mingle the colors you've chosen. Don't decide at the last minute to throw in a tube neutral or it will stand out like a sore thumb.