How to use the value returned by %St() function in WPS

[Frankenpod]

If I use %St(foreground colour) I can get the current foreground colour.  But %Vf(%St(foreground colour)) doesn't work, I guess because of something to do with cast or variable type.  I know %Vf(-) will achieve that, but I was thinking of maybe manipulating the value returned by %St in order to allow user to set one foreground colour and then have wps use variations of that colour in the theme, not just that exact colour.
Does anyone know if there any way to 'cast' the value returned by %St so as to to something useful with it?  Thanks.

[Frankenpod]

Just bumping this post in case anyone has an answer.

As it would be neat to be able to let the user specify the foreground colour (via the settings menu) and then to derive various shades of that colour, so you could pick 'blue' and the theme would adjust to use several different shades of blue, for example, rather than having to be monochrome.