Colour Expressions

I am a newby trying to change the colour of a poly line in a paint tool using an expression.
I appear to be able to define an expression for either the red parameter, or the green parameter or the blue parameter, but not for all three together.
Can someone please tell me how to change the colour using an expression?
Thanks
John

You have to use three expressions.

Chad
I have discovered the problem and I think it's a simple UI redraw problem.
When setting up a PolylineStroke within paint you can right click on the red value in the Apply Controls section and right click and select Expression you get a box to enter the expression. If you then do the same on the green value, you do not get a box. Same with blue. Therefore my problem of not being able to enter 3 expressions.
However if you then deselect the paint tool, removing it from the RHS and then select it again, then all 3 boxes are now visible and can be set them up as expected.
This behaviour does not happen with other tools.
John

Version 8.2.1 build 6