Change cell fill when cell is updated

I have a macro that runs indefinitely, and every 30 seconds (exact time will vary) it updates cell B1 of the active sheet (sheet name ABC). I would like the user looking at the screen to be alerted that the value has changed by changing the fill color of the cell. Is there a way of having the fill color of the cell alternate between blue, green and yellow every time the cell is written to? (Light colors so that the black text is still legible.)

I update the cell using this code, which appears in several different places throughout the macro:

Application.Range("B1").Value = inst

Thanks