Paul,
I personally don't find AisleFX's tutes valuable. GeekAtPlay.com, IMO, is much better.

You should look at the leaf material in the function editor. There you will see how I used a blender node combined with an Objection Center node to subtly change the colors. You can change the scale of the noise node for creating more differences per instance.

To experiment, try editing the blender node so there's 0 amount coming from the regular texture, then edit the color function so it uses a rainbow color scheme, then you can really see how it works.

HTH,
Chipp