Looking good.
I do have a potentiometer, but not three.
You only need one.
You would read the value of the pot.
Then 'map' the reading to a reasonable range.
Then divide the values of all three R G B by the mapped value.
Maybe the mapping could be 0-1023 to 1-5
Example:
Let’s say the pot was set to center, the reading would be 512.
The let’s say you 'map' 512 to 2
Let’s say R=100 G=50 and B=80
Then after dividing R G B by 2 would give: R=50 G=25 and B=40
Therefore the same colour but less bright.