What you're asking doesn't really make any sense. There's no reason to specify an alpha value (which normally make colors seem transparent) when coloring the background, as there's nothing behind the background to see anyway!

tfguy44 is right, doesnt make much sense.
But there are different ways to create color gradients. one is to use the set() function.


could like like this for example :

1. 
   void setup() {
     size(500, 500);
     smooth();
   }
   
   void draw() {
   
     for (int i = 0; i<height; i++) {
   
       color c = lerpColor(#ffffff, #000000, i/float(height));
       for (int j = 0; j<width;j++) {
         set(j, i, c);
       }
     }
   }
   

Thanks for the answer, your actually both are right, sorry.

You can just draw a bunch of horizontal lines across the sketch's area, from top to bottom, changing a bit the stroke() color for each line.

Consider that the gradient will also accumulate in an aggregate drawing (no background call). Comment out the background call in the example below to see what I mean. There are different ways to draw gradients and place them on top. You could use the pixel array or a PImage/PGraphic. However in the example below I just use a beginshape-endshape with vertices that have different colors or technically only different opacity.

1. void setup() {
2.   size(500,500,P2D); // Gradient beginshapes only work in P2D, P3D and OPENGL
3.   stroke(255);
4. }
5. 
   
6. void draw() {
7.   background(0);
8. 
   
9.   fill(255);
10.   for (int i=0; i<50; i++) {
11.     ellipse(noise(i+frameCount*0.1)*width,(sin(i+3+frameCount*0.01)*height),5,5);
12.   }
13. 
    
14.   noStroke();
15.   beginShape();
16.   fill(0); // black for the two top points
17.   vertex(0,0);
18.   vertex(width,0);
19.   fill(0,0); // black and transparent for the two bottom points
20.   vertex(width,height);
21.   vertex(0,height);
22.   endShape();
23. }

Hello Folks,
thanks a lot. Problem is solved.
I made smoke.