Does the Brooks Cambium C17 Saddle rub off?
If you're at all concerned, just go for the natural colored cambium saddle, which I don't think is dyed, so it shouldn't be an issue with that one.  I have not had any issue with colors tranferring from the slate colored saddle to my lighter colored synthetic shorts. However, After riding the slate colored saddle, the gray faded and now appears more brown.  So I would avoid the slate colored saddle if you're concerned. 

I actually had the reverse problem using the natural colored cambium saddle which I was demoing from a local shop.  The dye in my dark indigo colored jeans transferred to the saddle and turned it somewhat blue.