Today, the middle wire is usually red (+4.8 to 6V). That way, getting the plug reversed doesn't fry the servo. That system is almost universal in the USA and Asian (not early Japanese) servos. The Japanese converted several years ago, but some European (German) servos maintained center wire as ground --maybe they still do.
In any event, signal is never red or black. It is usually yellow, orange, or white.
John
In any event, signal is never red or black. It is usually yellow, orange, or white.
John