THREE.js fails to apply Gradient colors to Imported OBJ file

I'm trying to load and color a sample file from the THREE.js GIT called "WaltHead.obj" located here:

https://github.com/mrdoob/three.js/blob/dev/examples/models/obj/walt/WaltHead.obj

I'm able to load it into my project, but when I try to paint it with gradient colors using code that has already worked 100% for me with GLTF/GLB models - it doesn't work with this ".obj" file.

I made a fiddle to make life easier - but for some reason the model isn't loading into the fiddle file:

https://jsfiddle.net/gilomer88/tp2hkxne/27/

Hopefully someone can fix it so it loads?

Either way, like I said, I'm trying to color it with gradients - but it's showing up totally black. The code is in the fiddle, and also here:

const loader = new OBJLoader();

loader.load( "https://github.com/mrdoob/three.js/blob/dev/examples/models/obj/walt/WaltHead.obj", function ( theScene ) {
       console.log("  =>'theScene' = ", theScene);

       theScene.traverse(function(child) {
       console.log("\n==>Traversing the Scene now...");
          if(child.isMesh) {
             console.log("1. 'child' - FULL MESH INFO = ", child);
      
             if(child.name) {
                console.log(" 2. 'child.name' = ", child.name);
                console.log(" 3. 'child.geometry' = ", child.geometry, "\n");

                if(child.name.startsWith("Mesh_Mesh_head")) {
                   // Calling a convenience method I made here - code below:
                   let theColors = makeSmoothGradientColorsForMesh(child, "orange", "white");

                   child.geometry.setAttribute("color", new THREE.Float32BufferAttribute(theColors, 3));     
                   child.material.vertexColors = true;
                   child.material.flatShading = false;
                } 
             } 
          } 
       }); 
  
      scene.add(theScene);
      theScene.position.set(0, 0, 0);
    }) 

Here's my convenience method:

function makeSmoothGradientColorsForMesh(theMesh, c1, c2) {
   console.log("\n\n===>In 'makeSmoothGradientColorsForMesh()'!\n");
   console.log(">Incoming meshObject: \n", theMesh);

   let meshGeometry = theMesh.geometry;
   let meshMaterial = theMesh.material;

   // Do the Geometry-Dance:
   const positionAttribute = meshGeometry.getAttribute("position");
   console.log(">positionAttribute = \n", positionAttribute);
   meshGeometry.computeVertexNormals();

   // "BoundingBox" business:
   meshGeometry.computeBoundingBox();
   const aabb = meshGeometry.boundingBox;
   console.log(">aabb = \n", aabb);
   const f = aabb.max.z - aabb.min.z;
   const vertex = new THREE.Vector3();

   // COLORS business:
   let randomColor = new THREE.Color();
   let colorsArray = [];
   const c = new THREE.Color();                     

   console.log("positionAttribute.count = ", positionAttribute.count);

   for(let i = 0; i < positionAttribute.count; i++) {
      vertex.fromBufferAttribute( positionAttribute, i );
      c.lerpColors( c1, c2, ( vertex.z - aabb.min.z) / f );
      colorsArray.push(c.r, c.g, c.b); 
   }

   console.log("\n\n-->Will be returning the following colors:\n");
   console.log(colorsArray);

   return colorsArray;
}

For some reason, the colors coming back from my makeSmoothGradientColorsForMesh() method, return as follows:

Array(145440) [ NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, … ]

Your c.lerpColors() implementation is wrong. You're trying to pass a string into that method ("orange", "white"), which it won't recognize and return NaN.

That method is expecting two THREE.Color objects, so you should initialize those first:

// We use the strings to init our THREE.Colors
let color1 = new THREE.Color(c1);
let color2 = new THREE.Color(c2);

for(let i = 0; i < positionAttribute.count; i++) {
  vertex.fromBufferAttribute( positionAttribute, i );
  // Now we can use them for color lerping
  c.lerpColors( color1, color2, ( vertex.z - aabb.min.z) / f );
  colorsArray.push(c.r, c.g, c.b); 
}