Discoloured feathers

[Robber]

Two of my three Light Sussex Bantams exhibit a brown discolouration on their backs. Could it just be mud?

My red hens are clean and bright, as is the other (larger) LSB, who is a nice pure white.

Perhaps I should try washing it off!?

Pic attached.

 

[BantamLover21]

I would try washing it off, to see if it is just mud/dirt. However, it is likely that it is their normal feather color. The brown feathers aren't a desirable show trait (and I wouldn't use such birds in breeding), but they aren't a sign of a health problem.