I'm working on an app based on Apple's GLPaint sample code. I've changed the clear color to transparent black and have added an opacity slider, however when I mix colors together with a low opacity setting they don't mix the way I'm expecting. They seem to mix the way light mixes, not the way paint mixes. Here is an example of what I mean:

![enter image description here](https://i.stack.imgur.com/FIWJ9.png)

The "Desired Result" was obtained by using glReadPixels to render each color separately and merge it with the previous rendered image (i.e. using apple's default blending).

However, mixing each frame with the previous is too time consuming to be done on the fly, how can I get OpenGL to blend the colors properly? I've been researching online for quite a while and have yet to find a solution that works for me, please let me know if you need any other info to help!