This is just one way for you to color an eye, and you can use any color to do it. if you need more help just ask!
Step 1- lay down base color
Step 2- use a darker color and add a pupil and some shading
step 3- pick the lightest color of your chosen color and add some highlights
step 4- Pick a light grayish color of the one you are using and add some shading, then set the layer to multiply
step 5- make a white half circle, then set the layer to overlay
step 6- make some white dots and your done!
EDIT: This is not THE way to draw an eye, it just how to color one. Also once you get it down, you can add your own variations to this!
Depending on the eye color, it looks best if you have a good contrast between the light and dark shades. Also once you get the basics down, dont be afraid to add your own spin on it!