The color of a person's eyes is predetermined by that of his parents. In other word, it is genetically determined. Nowadays, medical science has no way to change the color of eyes. If you really want to get a purple eyes, use contact lenses is the only way you could choice. But there have some studies showed, human eye may be change its color by some disease or injury. However, what color it will be change, it's determined by God.
In order to get purple eyes naturally when you have the brown eyes, you could wear the purple colored contact lenses which will make your eyes get moisture and natural. At the same time, your eyes will look big and bright. You could have a try. There are a lot of colored contact lenses in the online store which will be suitable at the price. You could go and buy one.
Well, the easiest way you can do is to change what you wear. As our eye color can be changed with the light reflective and the light scattering frequency. Try some bright color shirts, particulary when you have a fair skin tone. As well as the makeup style. Relatively, you can take colored contact lenses into consideration, there are various kinds of lenses on the market with different colors, certainly your favourite purple. Besides, you can head to an eye office to see if they have some trial lenses, then you can see how they look like on you. Good luck.