adobe knuckles

His hands that day (the day I noticed mine) were on each taut
blue thigh. Most of the time they shook badly and he just let them
shake in his lap, but that day he was holding them still. The effort
to keep them from shaking turned his adobe knuckles white.

A Manual for Cleaning Women by Lucia Berlin

What does adobe mean in this context?
Does it mean sun-dried?
Or does it refer to color?

Please help. Thank you.

I think it refers to the action of the sun, and colour, and probably also texture, and perhaps also antiquity or tradition or craftsmanship. Imagery has multiple meanings.

I understand it to mean that they were normally the color of adobe.

Thank you！