Is it possible to have a checkbox set up so that when it is checked, it displays
a drop down menu containing various sliders, labels and a colour dialog box?
No. A drop down menu can only contain textual items. You can unhidden a section below the check box tough.
. I'm new to 2. using Mel.
Stop concentrating on GUI building, concentrate on the rig/functionality of your script*. IE what makes it tick, build it with node network and make custom attributes locators etc as appropriate and let maya make the GUI for you. Because onec you start thinking gee i want to make gui that look like this you will fall so deep into how to deploy things and such, you really don't want to waste your time here UNTIL you have all other things fixed and OWN the users pipe. Otherwise dont bother.
Think automating a rig. Besides a rig builds a gui, thats persistent with how maya works anyway and you can then animate it at later time. I get it your gui bound so you want to build guis but best tools are these that build no gui, but rather do all the rest automatically because they need no gui.
So learn to do the stuff first, maya actually builds your gui if you did it right. Thisway you dont loose the gui, which is bound to happen untill yoru VERY exprerienced. Besides maya users tend to dismiss the guis you build very quickly and concentrate on the channels you supply anyway.
- this isnt exactly true outside maya. but at your development stage and inside maya it holds true most of the time.