Check out the reference and search the forum. There is a lot of info available to help you get started. You can use an array of colors. You can use a timer to keep track of the time.

Code Example

1. // create and array of colors, in this case 2
2. color[] colors = new color[2];
3.  
4. // int to keep track of the current color
5. int selectedColor;
6.  
7. // int to keep track of the time since the last color change
8. int timer;
9.  
10. void setup() {
11.   size(500,500);
12.   noStroke();
13.   smooth();
14.   // set the colors in setup
15.   colors[0] = color(255,0,0); // red
16.   colors[1] = color(0,0,255); // blue
17. }
18.  
19. void draw() {
20.   background(0);
21.  
22.   // every 2 seconds
23.   if (millis() - timer > 2000) {
24.     selectedColor++;
25.     timer = millis();
26.   }
27.   
28.   // set the fill color keeping the increasing number within the possible list
29.   fill(colors[selectedColor % colors.length]);
30.   float radius = width/2 + noise(frameCount*0.005) * width/2;
31.   ellipse(width/2,height/2,radius,radius);
32. }

Well, thank you very much for this clear and excellent tutorial.

No problem. With regard to the last lines. You could set a fixed radius, but to make it a little more dynamic I used perlin noise running of the frameCount to continuously change the radius of the circle. To keep the colors within the list I used modulo. All of these functions can be found in the reference, check it out.