Change color of selected messages

Utilizing the message box’s bgcolor attribute, it's possible to change a message’s color in when it is selected, so as to indicate which message is currently selected.

Each time the user clicks on one of the four message boxes, the previously-selected message box is set to gray, the new number of the newly selected message box is stored (as the new "previously-selected" one), and the color of the newly selected message box is set to red. The route object routes the bgcolor messages to the desired objects.