This is a small group node which reproduces the color balance ASC-CDL node without the limitations of the current UI. It's quite handy to use this way as you can select 3 sliders in one clic&drag and then enter a value on your keyboard or move horizontally the mouse (use shift as a modifier key to do smaller increments). It is based upon the documentation provided by "The American Society of Cinematographers" about the "Color Decision List", and it adds the desaturation parameter of the 1.2 release. Please if you have a look inside the nodegroup, do not modify the values in the math nodes as they are precisely conforming to the documentation.
If you don't know what does the ASC-CDL, please have a look here
If you don't feel the need for this node, please have a look here