How to do this FadeOut Effect with openGL on iPhone?

Question

Hello,

I'm playing around with the GLPaint Example from Apple.

But I don't know how to create an effect which fades the already drawn stuff out. I created an example in Flash which shows the effect I'm looking for: http://staging.rwichmann.com/openglexample/

In Flash I'm drawing a texture on a BitmapData and in every frame I'm adding a ColorTransform to the BitmapData which fades out the old drawn data.

I guess there must be a similar solution in openGL. Something with the renderBuffer or frameBuffer but I didn't find any solution.

Do you have an idea, tip, hint?

Thank you, Raphael ;)