You don't understand the reason for the questions (which is also the reason why the first respondant showed you that monster).
12 volt doesn't tell us anything.
You probably do not want to control the voltage, you want to control amperage.
To be sure about that, we need to know how your bars are built.
How many LED's are in there and how are they hooked up.
Are all LED's of the same color in parallel (all A's together and al K's togehter), or are they series connected (K of LED 1 connected to A of led2, K of 2 to A of 3 and zo on), or any combination of these two possibilities ?
What resistors are connected to the LED's ?
Controlling the brightness of LED's should not be done by a potentiometer, if that is your goal.
There's a much better way to do this, certainly if you're using an Arduino.