Background Colour of Windows in MaxGUI?

So you already had panels/gadgets with the window as parent? Then make this new Panel the parent for all. (As in: first make this new panel, then proceed with the rest of the app)

local window:TGadget=CreateWindow(..)

local windowpanel:TGadget=CreatePanel(0,0,clientwidth(window),clientheight(window),window)

' list of your own application stuff, buttons, panels, etc.

local MyGadget:TGadget=CreateWhatever(.. .. ,windowpanel)
local MyGadget2:TGadget=CreateWhatever(.. .. ,windowpanel)

etc.