How can i make the colored part of my eyes bigger?

You naturally cannot: The colored part of your eye is what is termed medically, anatomically as your iris. It has the same size that it will always have from birth. This natural size cannot be changed unless you have some sort of injury to the eye that generally makes the iris abnormally shaped and smaller. The only possible way to make the color of your eyes larger would be to wear a contact lens with a larger iris.