I remember having hippeastrums that had roots coming to the surface, and those roots turned green wherever light touched them.

Also, seedlings floating in water developed a green hue on their roots, although it was less green than the leaves.

That was a while ago, but recently I Googled "hippeastrum green roots" and found some pictures of hippeastrums grown as epiphytes with roots that were also very green, as they had been exposed to light for a long time. The grower, in the photo caption, said that the green was algae. I'm not so sure about that, since it has also happened to me in conditions that were pretty dry. There was a thin white crusty "skin" produced by the root due to dryness before the spot where it re-entered the soil, but it was clearly green under that skin.

Does anyone else have experience with this?

Obviously the plant does not derive a huge amount of energy from root photosynthesis but it may just be a feature of all the tissue (except petals) in many plants to turn green when touched by light. It might be more complicated, evolutionarily, for a plant to turn that feature off, since an undifferentiated cell in a tissue culture is certainly green - and most plants simply have subterranean roots that do not see light, or roots coated by bark where they touch air, so it doesn't matter whether the underlying tissues can turn green or not.

The bulb surface itself will certainly turn green if you peel away the dry layers and let it contact light.

I have also gotten green roots in spider plants, some orchids are well known to have photosynthetic roots, and some kinds of willow that grow here will produce an intense pink pigment in roots that are exposed to light (I first saw them coming up like a carpet from a river bed next to where a willow was growing, so I experimented by taking a cutting and exposing the roots to light, and indeed they turned pink within hours that light touched them and continued growing).

It's not a hugely significant aspect in growing hippeastrum, but it brings some interesting opportunities when it comes to ways to grow the plant as an epiphyte outside soil, as far as visual interest and novelty.