http://www.LittleWebHut.com
This GIMP tutorial demonstrates how to use a selective coloring technique to create an image where only a portion of it is colorized. GIMP version 2.8.2 was used for this video. This is a good tutorial for beginners.
Link to image of eye
http://www.littlewebhut.com/images/eye.jpg
GIMP website
http://www.gimp.org