Thanks, ricksza. If the discoloration was due to moisture, especially road splash, I'm puzzled about how it could have gotten into the rear brake hydraulic system past the seals. I've changed the fluid each two years per the manual. But it would be scarier if the fluid had gotten discolored because something in the rear brake system is disintegrating. The manual basically says to change the brake hoses and replace the rubber parts in the cylinders each four years, but that seems a bit much when I've had bikes for up to 20 years without changing anything in the braking systems apart from brake fluid, pads, and shoes.