I don't know if its the easiest.
I personally take metrics zone by zone, and then use one of the following to create a match:
1) Curves, working on specific channels, painting over the mask.
2) Paint over an empty layer in color mode.
Note however that "tones" come in 3 dimensions: Hue, Saturation, Brightness.
When you notice a color variation, you should consider the 3, and use the one or all in simultaneous that will lead you to your goal.