The photograph has been artificially colourized. Original here: https://www.bonhams.com/auctions/26078/lot/22/

This is why I despise artificial coloration.

Its fine as a filter I can privately turn on to delude myself. But when the pictures spread it reinforced our modern biases in the past

Ah, that's certainly what did it. Thanks!

What makes you think it's denim? I would say it looks quite similar to the trousers the others are wearing (especially the ones of Whitney Oates).

Solely going by the timeline described in [0] it's rather unlikely that these are denim.

[0] https://www.bbc.com/news/magazine-17101768

Probably some form of cloth pants. Stylish though, definitely works with that haircut.