Do you find that having set black and white neutral points in an image, especially with Levels, modifying a mid-tone value with curves, such as a skin color in a portrait introduces a color cast? This is not surprising as the pivot points for the colors are at the far ends of the curves so there is nothing to prevent shifts. To prevent the shift you simply need to “lock down” certain values so that the pivot points are away from the ends and extreme values are protected.
To try this, open a curves dialog and place an additional point on the curve one block up and to the right of the bottom black point (assuming you are using the finer ten block detail in the curves dialog). Do the same one block down and to the left of the white point.
Those values will be at approximately 24 at the bottom and 230 at the top end. Additional points above and below these values can be added to lock in specific values if needed, and there is no rule as to where you lock down the values. Additional points help to minimize the shifts as the two initial points simply create new pivot points. Once your critical values are locked down, mid-range value changes will have a considerably lesser effect on the black and white neutrals.
Bryson Leidich 