Render water-paint in iOS

Question

I have been working with OpenGL in iOS, and setting the colors with glColor4f(r,g,b,a) and then drawing my own color on a white UIImageView

Answer 1

Looking at the examples you posted, it looks like they are using a simple Gaussian Blur with a radius of double your brush size. This may be an incomplete solution, but it's at least the first level.