You mean like in the attached  picture? The easiest is to just set it all one color and then add a second layer in the other color and set the distribution via the environment tab (that's what I did here). The other option is to make a color map with the two colors and then use the altitude input in the function editor to drive the distribution. You can use a map filter node to set the exact level. If you want to do it that way, let me know and I can show you.

Ciao

TD