When you select a new color of a location or transition, the undo action does not force a redraw for the location/transition to show the color change. However, the undo has taken place as witnessed by going to the declaration and then back to the template.
How to trigger the bug:
1. Right click location/transition and select a new color
2. Click anywhere on the canvas to remove selection of location/transition
3. Undo using either Ctrl-Z or Edit->Undo
4. Witness that color didn't change back
5. Go to other template/declaration/system in the tree and back
6. Witness that color has indeed changed