While I dont think a 590 even needs a side saddle, or a bayonet for that matter, there will be a difference in color between the receiver and the saddle...but not so much that you cant get used to it.
As for a sling I had no problem finding a simple nylon web sling that fit it very well. This one is 52" long.
http://www.windhamweaponry.com/shopexd.asp?id=106