Highlighting a mesh triangle on mouse click

I would like to change the color of a mesh triangle when I click on it. The following code is not working. When I draw a debug line then the correct triangle is highlighted, but the color is not changed. What am I missing here?

void HighlightTriangle2()
{
    if (!Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hit))
    {
        return;
    }

    MeshCollider meshCollider = hit.collider as MeshCollider;
    if (meshCollider == null || meshCollider.sharedMesh == null)
    {
        return;
    }

    Debug.Log("hit.triangleIndex = " + hit.triangleIndex);

    mf.mesh = meshCollider.sharedMesh;
    Vector3[] vertices = mf.mesh.vertices;
    int[] triangles = mf.mesh.triangles;

    Vector3 p0 = vertices[triangles[hit.triangleIndex * 3 + 0]];
    Vector3 p1 = vertices[triangles[hit.triangleIndex * 3 + 1]];
    Vector3 p2 = vertices[triangles[hit.triangleIndex * 3 + 2]];

    Transform hitTransform = hit.collider.transform;
    p0 = hitTransform.TransformPoint(p0);
    p1 = hitTransform.TransformPoint(p1);
    p2 = hitTransform.TransformPoint(p2);

    //Debug.DrawLine(p0, p1, lineColor, duration);
    //Debug.DrawLine(p1, p2, lineColor, duration);
    //Debug.DrawLine(p2, p0, lineColor, duration);

    int v1 = hit.triangleIndex * 3 + 0;
    int v2 = hit.triangleIndex * 3 + 1;
    int v3 = hit.triangleIndex * 3 + 2;

    colorArray[v1] = Color.red;
    colorArray[v2] = Color.red;
    colorArray[v3] = Color.red;
    mf.mesh.colors = colorArray;
}

I am creating the mesh as follows. The mesh is a cube with 24 vertices. I am using a legacy VertexLit shader. The color is not changed though.

private void CreateMesh()
{
    Mesh mesh = new Mesh();
    mesh.Clear();

    List<Vector3> vertList = DoVertices();
    mesh.SetVertices(vertList);

    List<int> trisList = DoTriangles(vertList);
    mesh.SetTriangles(trisList, 0);

    List<Vector2> uvsList = DoUVs_TEST();
    mesh.uv = uvsList.ToArray();

    MeshRenderer mr = GetComponent<MeshRenderer>();
    mr.material = defaultMaterial;

    Color[] colorArray = new Color[mesh.vertices.Length];
    for (int k = 0; k < mesh.vertices.Length; k++)
    {
        colorArray[k] = Color.white;
    }
    mesh.colors = colorArray;

    mf.mesh = mesh;
}