Jeans that is dyed with unmordanted logwood dyes brown.
Jeans that is dyed with logwood mordanted with iron dyes gray.
Logwood, sumac, and several other period vegetable dyestuffs can be made to dye brown or tan, but they are not color fast or light fast. Meaning with exposure and use, the gray "fades" to weak browns and tans.
A long standing discussion in the Hobby centers around ignorance or misunderstandings of natural dyes.
For example, Once Upon a Time there was a notion that repro Federal garments had to be sewn with tan or brown thread because so many original garments were sewn with tan or brown lookng thread. What was missed was that they were original dyed blue with mordanted logwood as a cheap substitute for indigo and what we were looking at was not brown thread but rather once blue thread that had turned from blue to brown or tan over time.
In gleichem Schritt und Tritt, Curt Schmidt
Not a real Civil War reenactor, I only portray one on boards and fora.
I do not portray a Civil War soldier, I merely interpret one.