In a four-wire trailer harness, the green wire is for the right turn signal, the yellow wire is for the left turn signal, the brown wire is for the brake lights and the white wire is for the ground. Most basic trailers use a four-wire harness.
The four basic wires are found on all trailer harnesses, although the wire colors may vary depending on the manufacturer. There are also trailer harnesses with pin connectors for five, six and seven wires, which are used to power auxiliary functions such as back-up lights and electric brakes. Adapters are available so that a four-wire connector can be used to match the other larger connectors.