You can then use the Color Picker to move the mouse pointer to a true color, take a sample from the true colors and return to your previous picture which you then can color with the color you selected.

You are probably already familiar with the History of Photoshop, which allows you to undo up to 20 steps. Unfortunately, when you quit Photoshop nothing is saved in memory and this no longer works. However, there is a way to undo changes to color and tone after days, weeks or even months.

Here is how: next time you apply a change in tone of any kind (e.g. through options Levels, Brightness/Contrast, Curves, Hue/Saturation, Color Balance or other), do not select them from the menu. Instead, scroll to the bottom of the Layers palette and click on New Fill or Adjustment Layer. This is the half black and half white circle. You will see a pop-up menu from which you can choose what tonal change (or filling) you want to apply. You’ll see a new special layer in Layers palette with the name change of tone (e.g. Color Balance). When you save the document and open it again, the Color Balance layer will still exist. To edit the settings of the Color balance, double-click on it. To undo the action, drag the Color Balance layer to waste.