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GLPaint-based OpenGL ES Blending Issue

I am working on an application based on Apple GLPaint code sample. I changed the transparent color to transparent black and added a transparency slider, however, when I mix colors together with a low opacity setting, they don't mix as I expect. They seem to mix light mixes, not how the paints mix. Here is an example of what I mean:

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The "desired result" was obtained using glReadPixels to render each color separately and merge with the previous rendered image (i.e., using Apple's default blending).

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You need to write this code in a function called when the color changes. and every time you need to install BlendFunc.

CGFloat red , green, blue;
// set red, green ,blue with desire color combination
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(red* kBrushOpacity,
          green * kBrushOpacity,
          blue* kBrushOpacity,
                      kBrushOpacity);

To do more things using BlendFunc use this link.

Please indicate if it works or not. This works for me.

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