Passing Custom Vertex Attributes to Vertex Shader

Trying to understand how to pass custom per-vertex Attributes along to be used in the shaders in processing 3, but seem to be missing something. In processing

  s = createShape();
    for(int i = 0; i< totPoints; i++){
          PVector r = new PVector(random(width),random(height));
          s.attrib("pointIndex" , float(i));

Now I am trying to use the custom attribute “pointIndex” to (for instance, just to test) modify the vertex color.

uniform mat4 transformMatrix;

attribute vec4 position;
attribute vec4 color;
attribute float pointIndex;

varying vec4 vertColor;

void main() {
    gl_Position = transformMatrix * position;
    vec4 c = vec4(1.0,.1*pointIndex,1.,1.);
    vertColor = c;

But alas this does nothing. I have also tried attribColor() and I have tried looping over the vertices in a subsequent loop and using setAttrib() to assign them each directly, still no go.