UI question regarding SfxToolBoxControls

I am working on fdo45671 which concerns the use of 'split buttons' for font/background/border 
colours, split button meaning that you can either choose from the colour palette or that you can 
use the used color.
Presently, the last used colour is stored within the control (mLastColor) and in the shell (e.g. 
ScFormatShell, SwTextShell) where the execute function is.
I would like to obtain the last used colour from the control when the execute function is called. 
When a user wants to use the last used colour, the execute funtion is called with the SlotId, but 
without other arguments.
Does anyone know if and how the control can be addressed from the execute function (with the SlotId 
as key)? I can then add a simple GetLastColor() method to the control and remove all redundant 
lastusedColor stuff within the shell.

Thanks for any help :)

Winfried