You have to mix the three primary colors: Blue, Yellow, and Red.
brown and green
Mixing black and grey will result in a darker shade of grey.
A darker color of gray or a color mostly black, if you do it right. It also depends on what shade of gray you are mixing with the black.
When you mix black and yellow, you get the color green.
Mixing brown and black would result in a darker shade of brown. The black would deepen the color of the brown but not drastically change its hue.
When you mix red and green you get a kind of brownish color and when you mix it with a little bit of black you make it darker.
When you mix yellow and black together, you get the color green.
Black
You cannot mix anything to any color to get black because black is the absence of color.
Mixing black and orange will result in a darker shade of orange, leaning towards a burnt orange or rust color. The intensity of the black will determine how dark the final color appears.
The color will still closely resemble black.
Mixing black and gray together will result in a darker shade of gray. Gray is a mixture of black and white, so adding more black will darken the color.