Mixing Colors Program

Hello,
This is my first post to the forums so I hope I don't sound like a fool.

I have been working on a project for a local children's museum and I have run into a bit of a problem. The project is an interactive instillation that will allow the children to draw on a screen with using the primary colors and where they colors overlap they mix to make a new color.

I'm using three jit.lcds for each primary color then mixing them into a jit.pwindow where they will all be displayed. I have try a few different object trying to get the color to mix right but it doesn't seem to be working right. I was wondering if anyone had any ideas on how I might go about doing this or if it even seems possible.

I would like to get the three primary colors Red Yellow and Blue to mix correctly as if it was color pigment but I know since this is done on a screen that it will want to mix Red, Green and Blue and mix based on light instead. If anyone has any idea on how to mix them better using either RYB or RGB your impute would be helpful.

Here is a small patch I made,in Max 5, just trying a few different ways. Sorry it so messy and for any incorrect spelling.