TikZ keys: turn on a feature and set a parameter at the same time

Question

I have a custom shape defined using TikZ. This shape has an optional part; if this is to be drawn, the colour has to be specified.

So I'd like to be able to use the following syntax:

\node [MyShape] at (0,0) {}; % Draw the shape without the optional part
\node [MyShape, mycolour=blue] at (0,1) {}; % Draw the shape with the optional part in blue

I know how to use boolean keys to turn options on and off. I know how to use keys to pass colour parameters. But here I need to do both at once!

Answer 1

Amalgamating the comments, it seems that you want to use pgfkeys' notion of styles. These can take arguments, so:

\tikzset{
    /tikz/mycolour/.style={
         colour of optional part=#1,
         draw the optional part=true
    }
}

means that calling mycolour=blue sets both the colour of the optional part (assuming that is what /tikz/colour of optional part=<colour> does) and the flag to draw it (assuming that is what /tikz/draw the optional part controls).