The first method I showed them was using white to lighten and black to darken the paint. I used titanium white, but I did mix my own black rather then using a tube black. I think the difference between the dark areas of the two paintings would have been more striking if I'd used black from a tube (full disclosure here: I forgot to bring a tube of black). It's been a long time since I used white as my predominant lightener and it felt awkward. I had to really concentrate and fight my now-automatic color choices!
In the second painting I used yellow and orange to lighten the apple and only added white when absolutely necessary. For the dark areas, I used purple and blue. I had planned to use a more white-looking tabletop color, but I accidentally put a streak of that rusty peach color down and we all loved it, so I kept going with it.
While comparing the two paintings side-by-side, the students commented on how much more vibrant Apple 2 was, and they noticed that in Apple 1 the white mixed with alizarin crimson created a pink tone, rather than a sense of light striking the apple. As they were leaving class pondering all these color choices, one student half-jokingly said, "Can't we just go back to black and white? It's so much easier!"