Its really simple, first off you'll want to duplicate your "spine layer" just in case the new colored spine isn't perfect you can give it another shot.
In Photoshop:
Image => Adjustments => Gradient Map...Then a little colored box will pop up. Click the area where the colors are and a new little window will pop up. In this new window there are little selectors where you can choose a new color. You will only need three selectors (one on the far right, one in the middle and one on the far left).
Far Left (Dark Green)if you want to go into greater accuracy you can mimic my exact colors i use.
When you click the far left selector to choose a color you can enter color values (numbers).
R: 20, G: 61, B:18
Middle (light green)R: 71, G: 192, B: 82
Far Right (white green)R:196, G: 255, B: 184
That was kind of the advanced version but its not too hard to figure out
