When you put a new thermostat in is it easy to install do you just put the green wire were the g is and the red wire were the r is and so on?

IF you purchase a new thermostat, it will come with detailed instructions.

Every house may be wired differently, especially an older home, may not have the color wires that are used currently or in the very recent past.

I just put one in, my home is over 55 years old, and with the provided instructions, I was a simple install and is working just fine. Some of the better brands will even have a toll free number to call if you have problems and can't find answers in the printed instructions.