using variable colour in action: set filter

I'm making a game where I would like for players to be able to drag a set of rgb sliders to change the colours of various graphics. The Set Filter action seems to do what I would like, except that it doesn't seem to let me choose a variable colour, only a static one. Is there any way around this? Is there some function that would be better suited to this that I haven't found?

I've tried with masks, but I would run into the same problem, and anyways, I'd need to increase my images by ~33% and go through a lot of trouble to ensure their z order is correct, so that masks aren't colouring the wrong items.

In the action for setting the color filter you can right-click the Filter bar and select Use Expression. Then with the rgb() expression you can use variables for each color channel.

<img src="http://i52.tinypic.com/2mqryj8.png">

I actually have a tutorial on making color sliders (and other things) here: