If I understand your question correctly, the answer is very simple. You probably recall that any number times negative one also becomes negative. (
n * -1 = -n)
So whatever value you want to invert, just run it through a Math node set to Multiply, and set the other value to
In the following example, a Math node is being used to flip geometry along its X axis:
The concept is the same when dealing with color in Material nodes, and also Compositor nodes. So it might help you to look at those as well and compare. Although in those editors there is a handy Invert node that saves us time so we don't need to process the inversion of color or alpha channels using Math nodes. But if we were to use Math nodes, the same result could be achieved. You can compare the different node setups below:
In the example of the arrow, the range is
1, with the center being
In the example of the colors, each channel has a range of
1.0 with the center being
0.5. So here we use an additional Math node set to Add, to keep the values in a positive range. (If you try this on geometry such as the arrow, the geometry will scale to 0 and you will see nothing.) So the method of inversion will depend on the range of values you are trying to invert. I hope this makes it clear for you.
If it helps, you can play with the .blend of the images above.