I'd like to quote myself ( https://enicar101.com/sherpa-graph-mkia/ )
"Lately I also have seen some grey dials. Its not clear if this was a real dial color you could buy or just a faded black one. "
Next week I will get a grey dial, so I can compare both.
The tan color on the Bulang Ia looks good, but I am thinking that it probably was gray at one time and then subject to some environmental influence that caused it to fade to 'powdered chocolate' in a pleasing uniform manner. I don't think it would look nearly as attractive if the gladius hands were greenish or bluish.
With little to no sign of rust, perhaps it was a warm but dry environment with lots of light? Who knows?