I'm using GIMP2 and found Blue and Yellow are making gray. Red and Green are making gray too. I'm using layers (one per color) and first assumed opacity would get me Green or Yellow in the examples above. Grain Merge gives the right secondaries, but if the mask layer is over white, the color is desaturated at about half-strength; red becomes salmon-y or pink. My question is: What do I do to ensure a Blue layer results in Green when over a Yellow layer without surprise color distortions? Am I missing something obvious?
First of all, let me spam you with this answer. Generating a series of colors between two colors It is filled with wisdom n_n
Blue is the exact complementary of yellow, on this RGB model.
With pure yellow, you have R and G components, and no blue. With blue, you have no G or R.
Gimp is using a linear transition, (Number 1 on my other diagram) this is the color is passing thru the middle of the circle... Or the square on my example.
But Cyan is another primary color, so you can use several blending modes that will give you green.
With programs like Gimp, the truth is that you cannot simulate painting very well, but using multiply and a normal brush you can simulate something like markers.
If you want a program that simulates painting the best option is Corel Painter, but there are other options like MyPaint.