i am used to basic arduino and wonder why this loop get stuck at red fill and doesn't restart from green:
void draw() {
   fill(green);
   rect(25, 25, 50, 50);
   delay(500);
   fill(red);
   rect(25, 25, 50, 50);
   delay(500);
}

It doesnt work like that... just think of it as a list of commands that are executed before the final image/frame is shown to the screen.

So when you do it like you do, you draw a red over a green rectangle at the same position.

there are different ways to achieve what you want.
the easiest probably would be to use modulo http://processing.org/reference/modulo.html
where the first number is always the frames of one colorcylce and the second one the half of it (frames one color is shown, if you change it to 10 for example, red is shown 40, yellow 10 frames)


like this :

void draw() {
  fill(#ff0000);
  if(frameCount%50<=25)fill(#ffff00);
  rect(25, 25, 50, 50);  

you shouldnt use delay like you are used to use it in arduino