How do I program in VBA, the background color of a Form Control Button?

I have a Button labeled "Proportioning Alarm", when I click this button, I would like to change the gray background color of the button to green, for an indication that the alarm procedure is running. I also have a button labeled "Stop Prop. Alarm", when I click this button, I would like to change the green "Proportioning Alarm", back to its default color of gray.

"Proportioning Alarm" button runs the subrotine Proportioning_Alarm()

"Stop Prop. Alarm" button runs the subrotine Stop_Proportioning_Alarm()

I would like to add the VBA code to change the button to green in the subrotine Proportioning_Alarm() at the beginning of the subrotine under initialization section. That way the first time the button is clicked it will turn green and stay green until the "Stop Prop. Alarm" button is pressed or the alarm series has finished. In order to change the "Proportioning Alarm" button back to its default color of gray I would like to add that VBA code to the Stop_Proportioning_Alarm().

I have attached my worksheet and sub-routines and modules.

Thank you so much for your response!

redsab