What smart materials are good to use on my pizza box?

For my graphics coursework project I have been asked to design a pizza box that uses a smart material to show the temperature of the pizza when it is delivered. I have researched leuco dyes and I have decided to use this smart material. However I have not been able to find information on what form of dyes to use on my box. Can anyone give me more infomation on what dyes to use or any infomation i might find useful?
Thanks!

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).