VEX motors can plug into the motor controller either of 2 ways, with matching colored wires (red-to-red), or with opposite-colored wires (red-to-black).
How come it can plug in either way, and what's the difference?
When the motor controller is plugged in with matching colors, the motor will turn in its standard direction (counter-clockwise). When programming the robot, motor modules will use positive numbers (the “red” direction in the EasyC interface) for clockwise. When the motor is plugged in in the opposite way, the motor will now move in a counter-clockwise direction when the motor module has negative numbers (the “green” direction in EasyC). Up is now down, and down is up.
Why would one ever want to do this? Let's say there is a shaft that's lifting an arm that's so heavy, it needs 2 motors in order to lift it (see below). If both motors are set to, say, +100 power (counter-clockwise / EasyC red), they will be fighting each other, trying to spin the shaft in opposite directions simultaneously.
So instead, flip one of these motor controllers (image below) to opposite connecting colors (red-to-black), and the motor on the right is now going in the same direction as the motor on the left. In EasyC one can now program both motors to +100 power and they will work together. This scenario makes programming much simpler than having to remember which motor needs +100 and which needs -100.
But is it OK to just program the motor on the right to be -100 power? Yes, sometimes. Other times it's not possible, such as when the 2 motors above are connected together with a Y cable, and plugged into a single motor port on the cortex. Both motors in this scenario will receive the same power level, whether positive or negative. Without the Y-cable, one would need to occupy 2 separate motor ports and program each motor individually.
Using a Y cable, or when adding an extension cable so that the motor controller wires reach the cortex, say, one ends up plugging a 3-wire cable into a 3-wire cable. Here, there's only one thing to know: All 3-WIRE CONNECTIONS MUST HAVE MATCHING COLORS, or else they simply will not work (that goes for the 4-wire integrated motor encoder cables too).