darker rust is the more desired shade but you will find all shades of rust. The rust markings on other colors are usually lighter than on a black Doberman.
I see all shades of rust, but will say that most European lines do have a darker rust on a more consistent basis. I agree that sometimes it looks more mahogany to me than rust. The dark markings are pretty, but I do like to see a nice contrast between the coat color and the markings.... so IMHO they can be too dark.
That said - my Harvard looks to have dark rust markings, and his mothers rust markings are a medium rust.
Mother:
Son:
His father is a red and has nice rust also - but not as dark as a black dog:
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Dillon's Barclay Amy CD RN WAC CGC TDInc "Velma"
CH Cha-Rish A Moment Like This RN WAC CGC "Louise"
Fitzmar's Command A Minute "Harvard" (10 pts/both majors)
http://fitzmardobermans.freeservers.com/