These strings are wound with nickel plated steel and feature red silked ends.
 
GHS says:
 
GHS Strings released Bass Boomers® in 1975, and altered the course of rock and roll! Winding a nickel-plated steel cover over a hex core, Bass Boomers® have a distinct low-mid bump that is aggressive and bold. Equally suited for pick or fingerstyle, the special recipe of stainless and nickel-plate on the low E and B strings keep your sound defined and strong. Hear why millions consider Bass Boomers® as THE POWER STRING.
Why Should You Play Bass Boomers?
You're running from a rock gig to a studio session and need a versatile string
You enjoy a clear, articulate picked tone with plenty of low end
You're looking to bring a lot of low-mid punch to your bass tone