How to match the color of dual display

Hi,

I have iMac (27-inch, Late 2013), 3.5 GHz Intel Core i7, 32 GB 1600 MHz DDR3, NVIDIA GeForce GTX 780M 4096 MB main system

and additional 27 inches Thunderbolt Mac display.

My problem is i see different colors of same image in different monitor. How I can match the color of both screens.

Thank you.