How to use part of an image for a button

I am trying to create a region select type thing with a map of a country. Below is similar to the image I will be using. I was wondering how you would create each color(region) on the map do a different function, so basically make each color its own button. The image is currently in unity as a texture.

 

simple logic (but very time intensive to implement)
create for each color area colliders and name them different.
with onmousedown check with raycast which collider is hitted.

 

I take it you already have a texture that is color-coded - a unique color for each area. You could make it so that when you click on the normal texture, you would map the clicked location onto your color-coded texture, then read out the pixel's color at that location.