Every freight locomotive I've seen or used has MU cables that are red, at least in theory. They're usually some very dirty, dingy, faded shade of pink. Most of them are strung through a bracket at their center, and hung by a chain to the underside of the drop step. There should be one on each end of every locomotive, though they occasionally do go missing. Only one is needed to connect a pair of locos, so the other is kept with both heads in the dummy sockets on whatever loco it's attached to. As far as the color of the sockets (technically, the hinged cover on the socket), that depends on the railroad. On NS, they are usually all black, sometimes (if the engine is clean enough to be able to see it) with the word "DUMMY" on the dummy sockets. Other railroads I've seen paint the live socket red, yellow, or white, but I cannot recall seeing the dummy sockets painted anything other than the same color as the rest of the frame.
The 37-pin (I hope I've got the number correct, though it doesn't really matter for our purposes) MU jumper cable was introduced by EMD on their early E-units, and I think it predates even the FT. Other builders used similar but incompatible electrical MU systems, and some even used pneumatic MU hoses. The EMD system won out, probably in large part due to the sheer number of F units running around at the end of WWII and shortly thereafter, and now any locomotive designed to MU with another will have this system. As an interesting aside, Even EMD used pneumatic hoses in conjusction with the jumper cable, controlling things like sanding. The number of hoses has decreased over time, until today there are only 3, all having to do with the independant braking system on the engines. All the other MU functions are now handled by the jumper cable. With the advent of $%&#$!!! electronic brakes, I wouldn't be surprised to see all the MU functiones handled electrically before long.
