SceneKit selectively color a face by a certain index

My objective: given some index i and some UIColor c, make the face at that index turn into that color...

let vertices = // all of my vertices in the format of SCNVector3

let vertexSource = SCNGeometrySource(data: verticesData,
                                    semantic: .vertex,
                                    vectorCount: vertices.count,
                                    usesFloatComponents: true,
                                    componentsPerVector: 3,
                                    bytesPerComponent: MemoryLayout<Float>.size,
                                    dataOffset: 0,
                                    dataStride: MemoryLayout<SCNVector3>.size)


var colors: [SCNVector3] = vertices.map { SCNVector3(1, 1, 1) } // Make it all white color at first
colors[10] = SCNVector3(1, 0, 0)                                // Pick one at random and make it red
let colorSource = SCNGeometrySource(data: NSData(bytes: colors, length: MemoryLayout<SCNVector3>.size * colors.count) as Data,
                                    semantic: .color,
                                    vectorCount: colors.count,
                                    usesFloatComponents: true,
                                    componentsPerVector: 3,
                                    bytesPerComponent: MemoryLayout<Float>.size,
                                    dataOffset: 0,
                                    dataStride: MemoryLayout<SCNVector3>.size)

let elements = SCNGeometryElement(data: NSData(bytes: indices, length: MemoryLayout<Int32>.size * indices.count) as Data,
                                primitiveType: .polygon,
                                primitiveCount: indices.count / 4,
                                bytesPerIndex: MemoryLayout<Int32>.size)

let g = SCNGeometry(sources: [vertexSource, colorSource], elements: [element])

My 3D object renders correctly. It is all white color, as expected. The red face, however, doesn't appear. I see a hint of red, but it looks like SceneKit is trying to color around the vertices rather than the face those vertices form. How can I force it to just color the face/polygon created by those vertices? Thanks!

from SceneKit selectively color a face by a certain index