Hi PeterW,
Touche!!
I must admit, I hadn't really considered the HMI point of view. Although when programming SCADA I have on occasion used this method but I have always used a "toggle" function button, which equates to press it once and it becomes true, press it again and it becomes false. rather like a hardwired light switch - and as you have said it did turn a different colour to indicate which position it was currently in (as per the position of the light switch)so you knew from the colour/position what the next action would be. With a hard wired spring return pushbutton (we havn't been informed as to the type or any other visual aids) you may not be immediately aware of what action the button will enable the next time you press it.
My preference even on HMI / SCADA would still always be seperate buttons using the latch logic in the PLC to enable your actions.
Cheers,
Lee O