sony playstation2 query

Just got one of these(2nd hand ebay) for our foster kids bedroom. I know nothing about them. It works 100% with the games. When I play a dvd, it appears all green on the tv. A bit like when one of the"guns" goes on old tvs.You can still see the picture, & no, its not just on the incredible hulk dvd, before some smarty pants asks!!
Any idea what this is/how to rectify?
Cheers, matt.