In class 11/13

attribute vec4 attributePosition

varying vec3 vertexColor 
...

void main() {
     float x = 0.0;
     float y = 0.0;
     float z = 0.0;

     if (attributePosition[0] > 0) {
         x = 1.0;
     }
     ...
     vertexColor = vec3(x,y,z);
}