As for HOW to accomplish keeping the color balance in check when adjusting the curve channels independantly, here's one method:
You can tweak and adjust the individual channel curve points in any fashion you desire. This in and of itself will most certainly alter the color balance. HOWEVER, if you first make a layer, then make all the curve adjustments on the layer, you can change the blend mode of that layer to "luminosity" and the overall color-balance will remain the same as the original and only changes to the luminosity of the image will be made. (Alternatively, you can run the curves dialog on the main layer, but right after you hit "okay" go to "edit> fade curves" and here you can again set the blend mode to luminosity. )
Cheers,