Non-prescription Colored Contacts?

I was thinking about trying a pair of colored non-prescription contact lenses but…i dont see how they work. I know at the eye doctor you have to be fitted for contacts, I know that everyone’s eyes are shaped differently…some have an astigmatism…and so on. How can you order a pair that just comes made in one shape…and fit perfectly?
I was just wondering if anyone had any helpful tips from personal experience. I have green eyes (on the darker side) and I just thought it would be fun to get some to try out. I was gonna order some on online from hollywoodcolorcontacts.com but I emailed them a few questions that I had…and they wont email me back. I love my eye color-I just wanted to try ‘em since I always see adds for them online.
I have never worn contacts before and I have glasses that i wear only when i need to see long distances.
Any help or tips would be great!
Thank you!
xoxxx