LED Dimmer only using NPN BJT and potentiometer
It won't be very stable- the color and brightness will change as the transistor warms up. 

Here is a bit better circuit that is about as simple (uses a single quad op-amp LM324): 







simulate this circuit &ndash; Schematic created using CircuitLab

The voltage at the top end of the pots is about 450mV, so the maximum LED current is 0.45/18 = 25mA. Adjust R4 to suit your requirements. The 130 ohms is added since this circuit can supply enough current to burn out the LED and/or transistor. That could happen if the bottom end of R2 became disconnected, for example. It's sort-of optional, but probably worth putting in. If the LED brightness flattens at the high setting you may have to reduce it. 

You could also simplify it by combining all 3 R1's into a single 33K resistor (so 1 fewer part than your circuit). 



R1 and R2 provide a voltage to the non-inverting op-amp input of 0 to 450mV, depending on pot setting. The op-amp drives the transistor to draw current through the LED to reproduce that voltage across R4, so the current through the LED can be set from close to zero to about 25mA with the values shown.