Perhaps the question is no longer relevant. But my solution to this issue will probably help someone.
I used layers, that is, the ability to overlay images on top of each other and make DigitalValue transparent. At the top is DigitalValue as an input (writable). Underneath lies DigitalValue like a lamp (not writable). DigitalValue-inputhasOn and Off Image = transparent.png. The DigitalValue-lamp has On Image = green.png, Off Image = red.png.