The difference is that most dyes back then very natural or vegetable. Curt's post lays out the variations. The Federals tended to have "better" production facilities. Indigo, while a natural ingredient, does not have the same unstable properties as Sumac or other vegetable-based dyes. Hence Federal uniforms often look the same as they did 150 years ago (other than wear).
Originally Posted by Danny
As has been stated, purchase a good garment from an authentic accurate sutler, wear it in the field, and within a season you will look fine. My RD II jacket, for example, from Becky Nall's Cotton City Tailors continues to evolve
in color, yet always looks "right."
Treasurer, The Rowdy Pards
'In the end, it's the history, stupid. If you can't document it, forget about it. And no amount of tomfoolery can explain away anything that makes history (and living historians) look stupid and wrong."