hello,
you draw a rect with the command rect

e.g.

1. rect (200,110,40,50);

before it you use the command fill to set the fill color.

Since you want to change the color when mouse over it:
You use an if statement to check wether the mouse pos is within
the rects position.
When this is the case, chose a color, otherwise another one.

The positions when drawing the rect and checking the mouse on it must be the same of course (e.g. 200 and 110 etc.):

1. if (mouseX>200 && mouseX<200+40 &&
2. mouseY>110 && mouseY<110+50) {
3. fill (255,3,3); // red when mouse over
4. }
5. else
6. {
7. fill (3,3,255); // blue when mouse not over
8. }
9. // here comes rect
   

you can look up those commands in the reference.

show your code with this hints build into it and we will help further.

Greetings, Chrisir 

     

thanks for your helps. what if i can't to click rectangle to change color ? am i need to use mousePressed in these coordinates ?

If I understand you, then yes...
It would look something like this:

1. if(mousePressed && mouseX>200 && mouseX<200+40 && mouseY>110 && mouseY<110+50) {

yes, see

http://www.openprocessing.org/sketch/48351

or use a library such as controlP5

Also see Processing Examples > Topics > GUI

(as mentioned, for more advanced gui's there are contributed libraries such as controlP5)

I recommend you use the controlP5 I had to make some buttons, like you I was doing when I saw the controlP5 was all very easy and practical.

thanks for all of your helps and controlP5 is best solution for me :D