The green color is most likely caused to food change, if you have changed the diet. Illness is a possibility. Rats diets need to be changed slowly just like all animals so their body can get used to the change. If the diet is changed overnight with no grace period it can cause tummy problems. The new rat should not be introduced to Stewart for a minimum of 2 weeks. New rats need to be quarantined, housed far away from existing rats in case they carry any illnesses. It is advised that new rats are housed in a totally different place (like a friends house) and after holding the new rat washing exposed skin and changing clothes is also important, so the illnesses are not carried and given to the other rats. Introducing them might be a little tricky since they are un-neutered males (right!?) they will most likely fight, especially since Stewart has already claimed you and the cage as his kingdom.

Here are a bunch of articles that you should read on diets, quarantine, and introductions:
http://www.rmca.org/Articles/quarantine.htm
http://www.ratfanclub.org/newrat.html
http://www.ratfanclub.org/diet.html

I hope those articles answer some of your questions.