Changing material or colour in a Dynamic Component - how to apply 'none' or 'Default'?

I’m trying to extend my Stage Lantern dynamic component to allow selection of a light colour for the light pool.

I want one of the options to be ‘none’ - either the default colour, or a white set with opacity 0.

Should I create a colour white with zero opacity and name it Clear, instead, for example?

In my colour selector testing, I find I can edit an existing colour’s opacity and give it my own name, or use the name of an existing opaque named colour such as 0127_Magenta.

But it doesn’t work to assign the value Default to a colour in the pick list. I’d rather have that, I think, than a zero opacity white.

I get this error in the Component Attributes:

Does it matter - it appears to work to change the light pool to the default white.

Is it possible to select the default material some other way so as to avoid this error? If so, how?

Here’s my test model, saved back to v2017.
Colourparent.skp (35.1 KB)

Example in the Meterial section
Thickness must be Decimal number

  =if(Thickness=1.86,"orange",if(Thickness=1.76,"green",if(Thickness=1.8,"yellow",if(Thickness=1.7,"limegreen",if(Thickness=0.9," greenyellow",if(Thickness=0.6,"yellowgreen","red"))))))