Its to do with the history. Lets say I have two objects and I boolean them and the result is a third object. With the history on the two starting objects, that are now dissapeared into the third, will still be displayed in the outliner.

If you select the third object, it turns green. If you select one of the first two objects, they turn pink as they are now linked via the third objects history. You can edit the components of the first two objects, but you can still move them around etc and change the third object as a result. The pink selection colour indiscates that the objects are linked.