Models, nodes, properties and references are highlighted.
green means new items, blue means modified items, brown means unversioned items.
Highlighting appears for all of the editor tabs: for language aspect tabs of a concept and also for custom tabbed editors declared in plugin aspect of a language (see Plugin: Editor Tabs).
Every kind of changes are highlighted in MPS editor: changing properties and references, adding, deleting and replacing nodes.
If you hover mouse cursor over the highlighter's strip on the left margin of editor, the corresponding changes become highlighted in editor pane.
If you want to have your changes highlighted in editor pane all the time (not only on hovering mouse cursor over highlighter's strip), you can select "Highlight Nodes With Changes Relative to Base Version" option in IDE Settings → Editor.
If you click on highlighter's strip on the left margin, there appears a panel with three buttons: "Go to Previous Change", "Go to Next Change" and "Rollback".
If you click "Rollback", all the corresponding changes are reverted.
This feature allows you to feel free to make any changes to MPS model in editor without any fear, because at any moment you can revert it right from the editor.