Here we have a wonderful pair of blue eyes. They're fine as is, aren't they? Wrong. You have to change them to green. Why? Who knows, but I'll help you out in your project.
The first thing we're going to do is create a Hue/Saturation adjustment layer (Layer>New Adjustment Layer> Hue Saturation). However, at this point it won't do anything for us as changing the Hue will alter the entire image and we only want to alter the eyes.
So our objective is to select the eyes. To do this we're going to use the Pen tool. Go to the Paths tab (in the bottom right area, next to Layers and Channels) and create a new Path and then select the Pen tool. With the Pen tool, simply click to create a point. Click again to create another point. You'll notice that a line linking the two was created automatically. To get a round line (to match the curve of the eyes that we're trying to select) click anywhere on the line between the two points; this will create another point. Holding down the Ctrl key, simply drag the point until you get the desired shape. Continue creating points and dragging them until you have the whole eye outlined. To finish the Path, click on the point that you started with. Repeat this process for the other eye (make sure to do this in the same Path; do not create a new Path). Once you have both eyes selected, right-click on the Path layer and select "Make Selection".
Now go back to the Layers tab and select the Hue/Saturation layer. There should be two boxes on that layer, one with a few black and white rows in it and one empty, white one. Select the white box - this is the Mask for this layer. The Mask will allow us to only select the eyes. Press Ctrl+Shift+I to invert your current selection. Previously we had only the eyes selected, now we have everything except for the eyes selected. Navigate to the top of the screen and select Edit>Fill. Select 'Black' on the dialog that opens and hit OK. The white mask box should now be filled with black, besides for two white spots corresponding to the eyes.
We can now change the color of the eyes. Drag the Hue slider (located above the layers) along the colors until you find the color that you want. You may also want to drag the Saturation slider a little bit to the right to create a more vivid color.
As long as you save the image as a Photoshop Document (.psd.), you'll always be able to go back and change the color of the eyes to whatever you want, just by dragging the Hue slider.