For the same I have been digging into the files:
fill-and-stroke.(cpp+h)
paint-selector.(cpp+h)
fill-style.(cpp+h)
I could figure out all the details of how the FiLL and STROKE dialog box is drawn plus how it interacts with the objects. What I don't get is the code where the changes are actually applied to the selected objects on the canvas(desktop).
Facts I already know:
1.) in Inkscape, there are two representations of the document being worked on: Object and XML.
2.) Changes in the XML Representation is automatically reflected in GObject Representation but not vice versa.