Merging colors... How do it with Gimp?

[Krikor]

Hi,

We obtain the secondary colors by superimposing the primary colors, such that:
Red + Yellow = Orange
Blue + Red = Purple
Yellow + Blue = Green

Is there any way to produce this with The Gimp?

   

I laid each color in a layer and tried to get the secondary colors, I tried the layers modes and the composite mode, but without getting the expected results.

Thx!