The easiest, and probably best way is with the histogram in the Color Corrector:
(note: all balancing and work done in 16 bit Float--if you work in 12 or 8 bit you run a risk of clipping out detail if you are not careful)
Set your black and white points on the R,G, and B to balance your image by sampling an area for blacks, and entering those values into the IN on the black levels, then sampling an area for whites and doing the same with the IN on the white levels.
to un-balance to the original put the IN values into the OUT values and reset the IN's back to 0 and 1 respectively.
Using Gain and Offset (separately) in Color Correctors:
--Use only the master controls (no highlights/mids/shadows) and use only gain and offset.
--Make two CC nodes if you're going to use both Gain and Offset. Have the Gain CC come first.
--Balance your shot
--At the tail of your batch tree add two more CC's to invert the Offset and Gain--in that order.
To unbalance use inverse of the upstream offset on the downstream one and use the 100/gain*100 math/expression to get your inverted gain values and your image will be back to where it was.