Its done by taping the barrel prior to browning and/or bluing the barrel. You cut away the tape where you want the darker stripes, or pattern to appear on the barrel.
Then, you first brown that part of the barrel, or rust blue it, and then remove the remaining tape, and brown or blue that. The area that has been previously been browned or blued, will be darker, and you will have the desired finish.
Someone here, a year and a half or so back, put up pictures of a pistol he did, by " painting" the streaks or stripes on the barrel with a paint brush, using either bluing, or browning solutions. It was a fine piece of work. The concepts are the same. How you do it is all that differs.