How common is it for my eyes to change colour so drastically? i have brown eyes and my dad has one blue eyes one green eye (that appears yellow sometimes) why the rapid change?

Not common: Eyes generally do not change colour after the age of 6 months. Certain conditions, such as arcus senilis, can cause eyes to appear to become more blue or grey, and others, such as certain inflammations and glaucomas can cause a brown iris to turn blue. If there is a rapid change in your eye colour, see an eye doctor as soon as possible.