When I added a little 6DE to my 5ME formula, my final result became very brown and dull. Why?

When working with deposit only colors, the best results are obtained if the formulas are kept simple with less intermixing.

Again, it is important to remember that deposit only colors do not work like permanent oxidative colors.  When you mix a violet-based demi color with a yellow-based color, the result is brown (violet + yellow = brown).  When working with deposit only colors, the best results are obtained if the formulas are kept simple with less intermixing.