How can I colour things in OpenGL ES 2.0 based on their depth?

This is fairly simple to do if you just want to use the Z position of your geometry. You could have a vertex shader like the following:

attribute vec4 position;

varying float zDepth;

uniform mat4 modelViewProjMatrix;

void main()
{
    vec4 newPosition = modelViewProjMatrix * position;
    gl_Position = newPosition;
    zDepth = (2.0 - (1.0 + newPosition.z))/2.0;
}

and a fragment shader like

varying highp float zDepth;

void main()
{
    gl_FragColor = vec4(zDepth, zDepth, 0.0, 1.0);
}

which would produce the following look:

I have an example of this within this iPad sample code I assembled for my OpenGL ES 2.0 class.