Answer derived from WestLangley's Fiddle here: http://jsfiddle.net/ST4aM/2/.
Vertex Shader
<script type="x-shader/x-vertex" id="vertexshader">
#ifdef GL_ES
precision highp float;
#endif
varying float x;
varying float y;
void main()
{
gl_Position = projectionMatrix * modelViewMatrix * vec4(position,1.0);
x = position.x;
y = position.y;
}
</script>
Fragment Shader
<script type="x-shader/x-fragment" id="fragmentshader">
#ifdef GL_ES
precision highp float;
#endif
varying float x;
varying float y;
void main()
{
if(x > somePosition)
{
gl_FragColor = vec4(0.0, 1.0, 0.0, 1);
}
else
{
gl_FragColor = vec4(1.0, 0.0, 0.0, 1);
}
}
</script>