Buy
solid black for the mainsheet.

Why?

Scott,
I like solid black for the mainsheet because it's always being
handled and it lays on the cabin sole and gets stepped on. After all
that it still looks good. Anything else looks crappy after a season.
I also use black for the outhaul and Harken jib cars. That jib car
line is always getting stepped on too.
Everything else is color coded.