Changing the background color is as simple as using a different color when you call background().

This post is in response to the message you sent to me regarding handling option selection events when using the GUI builder tool. With regard to resizing windows etc. then I am sure others can add to this thread.
--------------------------------------------------------------------------------------------------------------------------------------------------

I used the GUI biuilder tool to create the simple demo application shown listed

There are two option groups:-
Group 1 has three option buttons to select the colour and
Group 2 has two option buttons to select light/dark.

In the builder tool I canged the variable name and  event method name to make it easier to identify the event handlers. They are :-

Variable name      Event Method name
optRed             option_red_select 
optGreen           option_green_select 
optBlue            option_blue_select 
optDark            option_dark_select 
optLight           option_light_select 


The five event methods are at the start of the gui tab and these are executed when the corresponding option is selected.

Main Sketch Tab:

1. // Need G4P library
2. import guicomponents.*;
3. int backCol;
4. int r,g,b;
5. boolean dark = false;
6. 
   
7. void setup(){
8.   size(480, 320);
9.   createGUI();
10.   customGUI();
11.   // Place your setup code here
12.   r = b = 0;
13.   g = 250;
14. }
15. 
    
16. void draw(){
17.   background(r,g,b);
18.   fill(200);
19.   noStroke();
20.   rect(0,0,240,100); 
21. }
22. 
    
23. // Use this method to add additional statements
24. // to customise the GUI controls
25. void customGUI(){
26. }

gui tab

1. /* =========================================================
2.  * ====                   WARNING                        ===
3.  * =========================================================
4.  * The code in this tab has been generated from the GUI form
5.  * designer and care should be taken when editing this file.
6.  * Only add/edit code inside the event handlers i.e. only
7.  * use lines between the matching comment tags. e.g.
8.  void myBtnEvents(GButton button) { //_CODE_:button1:12356:
9.      // It is safe to enter your event code here 
10.  } //_CODE_:button1:12356:
11.  
12.  * Do not rename this tab!
13.  * =========================================================
14.  */
15. 
    
16. void option_red_select(GOption opt_selected, GOption opt_deselected) { //_CODE_:optRed:589177:
17.   r = g = b = 0;
18.   r = 250;
19.   if (dark == true)
20.     r = 125;
21. } //_CODE_:optRed:589177:
22. 
    
23. void option_green_select(GOption opt_selected, GOption opt_deselected) { //_CODE_:optGreen:689373:
24.   r = g = b = 0;
25.   g = 250;
26.   if (dark == true)
27.     g = 125;
28. } //_CODE_:optGreen:689373:
29. 
    
30. void option_blue_select(GOption opt_selected, GOption opt_deselected) { //_CODE_:optBlue:669478:
31.   r = g = b = 0;
32.   b = 250;
33.   if (dark == true)
34.     b = 125;
35. } //_CODE_:optBlue:669478:
36. 
    
37. void option_light_select(GOption opt_selected, GOption opt_deselected) { //_CODE_:optLight:808969:
38.   if (r == 125)
39.     r = 250;
40.   if (g == 125)
41.     g = 250;
42.   if (b == 125)
43.     b = 250;
44.   dark = false;
45. } //_CODE_:optLight:808969:
46. 
    
47. void option_dark_select(GOption opt_selected, GOption opt_deselected) { //_CODE_:optDark:825304:
48.   if (r == 250)
49.     r = 125;
50.   if (g == 250)
51.     g = 125;
52.   if (b == 250)
53.     b = 125;
54.   dark = true;
55. } //_CODE_:optDark:825304:
56. 
    
57. // Create all the GUI controls.
58. // autogenerated do not edit
59. void createGUI(){
60.   G4P.setColorScheme(this, GCScheme.BLUE_SCHEME);
61.   G4P.messagesEnabled(false);
62.   optGroup1 = new GOptionGroup();
63.   optRed = new GOption(this, "Red", 32, 8, 90);
64.   optGroup1.addOption(optRed);
65.   optRed.addEventHandler(this, "option_red_select");
66.   optGreen = new GOption(this, "Green", 32, 32, 90);
67.   optGroup1.addOption(optGreen);
68.   optGreen.setSelected(true);
69.   optGreen.addEventHandler(this, "option_green_select");
70.   optBlue = new GOption(this, "Blue", 32, 56, 90);
71.   optGroup1.addOption(optBlue);
72.   optBlue.addEventHandler(this, "option_blue_select");
73.   optGroup2 = new GOptionGroup();
74.   optLight = new GOption(this, "Light", 136, 24, 90);
75.   optGroup2.addOption(optLight);
76.   optLight.setSelected(true);
77.   optLight.addEventHandler(this, "option_light_select");
78.   optDark = new GOption(this, "Dark", 136, 48, 90);
79.   optGroup2.addOption(optDark);
80.   optDark.addEventHandler(this, "option_dark_select");
81. }
82. 
    
83. // Variable declarations
84. // autogenerated do not edit
85. GOptionGroup optGroup1;
86. GOption optRed;
87. GOption optGreen;
88. GOption optBlue;
89. GOptionGroup optGroup2;
90. GOption optLight;
91. GOption optDark;

Hope this example helps