It looks good to me. The contrast works because he appears to be under a tree. The contrast comes off as the dappling of leaves and shadow, rather than an error on the artist's part. I like the grey hairs that peak out here and there. The hair is well done, it feels as if I could literally run my fingers through it.

However, the robe feels a little less defined. Also the tree in the far back, it could use a little more care. I'm not saying it should be sharply outlined, but at least a little more like the tree in the foreground. The one in the back is coming off more as a green cloud than leaves.

Keeping in mind that it was a speed painting, the overall impression is nice, and the hair really catches the eye. But you really could have used a little more detail on that far tree. It detracts a bit from the painting, more than the contrast I would say.