There's two ways to do this, one way is to burn off layers of melanin with a laser, changing from brown to green to blue, but there is a high risk of eye damage or blindness, but it really good depending on the doctor, still keeps the natural pattern on the eye. Then there's this way which is basically a tattoo so it's flat, no pattern, and they have to leave a rim (better doctors do a thinner rim) of the original colour. It's uncanny valley and you can get much better coloured contacts. Also still comes with risk of eye damage and blindness just slightly less but mostly it's cheaper.