Change the eye color with The Gimp - Before.

And after.

Step 1: Choose the eye color.

Open the photo where you want to change the eye color of in The Gimp.
Change the foreground color to the color you want the eyes to get. In this tutorial 28c7d5 is used.

• Click in the Toolbox on the forground color.
• Type the value 28c7d5 in the HTML notion, or choice an other color.

Don't worry about the eye color to much. Just pick a color that is close to the color you want. You can change the eye color easily later and it's easer to get the right color later.
Create a new layer: Layer > New Layer
Change Layer Fill Type to Forground Color.
Now you have a blue colored photo, don't worry that is good.

Step 2: Some basic steps to change the eye color/

Duplicate the background layer:
Activate the background layer in the layer palet.
Click the “Create duplicate” button at the bottom of the palet.

Drag the duplicated layer in the layer palet so it stands on top of the layer stack.

Add a layer mask to the duplicated layer.
Go to the layers palet > Right click on the duplicated layer and choose Add layer mask > Option:White.

Reset the foreground and background color in the Toolbox.

Activate the Paintbrush tool in the Toolbox. Choose a hard, large brush (the exact size depends of the image you use).
Brush over the eyes, but don't go close to the edge of the eyes.

Now choose a soft, small brush and brush over the egde of the eyes.

If you brush to much away change the foreground color to white to bring it back.

Step 3: Give the eyes there new color.

Activate the colored layer.
And change the blend mode to Color.

And the new eye color starts to appear.

Step 4: Improving the eye color.

To get the best result you need remove the color of the background layer.
Activate the background layer and go to Color > Desaturate.
Set the option Choose shade of gray based on to Luminosity.
Sometimes Lightness or Average gives a better result.

Step 5: Give the eyes a dark edge.

This step isn't always needed.
Natural blue eyes have a darker black line at the edge. In this step you'll draw this line.
Create a new transparant layer:
Activate de black and white layer (this way the new layer will be created on top of the black and white layer)
Click the new layer button and set the Layer fill option to transpanrcy.

Activate the brushtool in the Toolbox.
Set the foreground color to black.
Choose a small, soft brush.
Draw a black edge around the eyes.
If you've missed activate the Eraser tool to erase the bad parts.
Set the opacity to around 50%.

Tip: It offen gives a better result if you draw the lines of each eye on a seperate layer. So you can varia the opacity.

Step 6: The last tweak to finesh the color change.

This step will give the blue eyes there natural light.
Go to Color > Curves.
In for the image in this tutorial this curve is justed:

And that gives this as a result

Finally, activate the colored layer and drop the opacity to 90 to get a natural eye color.

If you are not happy with the color:
Change the forground color
Activate the colored layer
Choice Edit > Fill with FG Color