I've learned that changing the color of your eyes has surprisingly low impact to your appearance. This is because people mostly don't actively notice the color of your eyes. I mean, it may be in the back of their head, especially if you're a model or something. I've gone from brown to black, and brown to gold, with no one noticing.
But what about a costume? I'd say making your normally colored eyes another normal color is worth about 1% of the total costume. Not worth it even a little. Now if your trying to look like someone with bright red eyes, or mirrored, that's different. There are also sclera that cover all visible parts of your eyes. These types of change are very necessary.
But there is another type of change that is very necessary. Some characters don't wear glasses. In fact, most don't. Venom, for example. Last year at Convergence my new costume was Benny from Fallout; New Vegas, and I wore it with my glasses.
The problem is I don't have a prescription for contacts. The reason is, I don't need one. I think I look better in glasses. And even if I wanted them, there's no need for a special prescription for them. Corrective lenses need to be less specific as they get nearer to your eyes. The prescription of your glasses is more than plenty unless you have a weird curve to your eyes, which I don't, and that does not change.
The United States Daycare Service disagrees. They classify contacts as drugs for some reason. To be clear, if you get the wrong contacts, you say, "These feel uncomfortable, I think I will take them out." But this is the world I live in.
Last year my plan was to give ten online contact sellers my regular glasses prescription, and hope one didn't notice. This plan was so successful that I had to wear my glasses to Convergence last year.
This year, I did fifteen minutes of poking around the internet, and found a place that ships to the US, and doesn't ask to be sent a copy of your prescription. This was a better plan.