Variables using "random()" change everytime they are called

I am drawing a colored object with random RGB color values via the background() function in Processing, waiting half a second, and then changing the position of the RGB values (e.g. swapping R with B, etc):

void draw() {
    z = float(random(255));
    x = float(random(255));
    c = float(random(255));
    background(z, x, c);
    delay(500);
    background(c, x, z);
}

However, the way I am declaring the color values randomly doesn't seem to be constant within the function, despite storing the values as variables; rather, the second background call is generating entirely new values for the RGB variables.

I want the z, x, and c variables to not change upon being called into the 2nd background() command, but can't quite figure it out.

I tried making other variables, using frameRate(1), and moving the delay(500) command past the 2nd background(), but unfortunately the variables change upon being called the 2nd time, and I want them to keep their value until the next void draw() loop.

Even with the delay() call (which I don't recommend), the second time you call background() you'd overwrite the values set during the 1st call.
You probably want to change the colour once per frame (not twice).

If you'd like to toggle between the two calls ((z,x,c) / (c, x, z)) you can use a separate boolean value or expression to swap between the two (thus avoiding changing the background twice in the same frame).

For example you can check frameCount and the modulo(%) to check if when frames alternate.

e.g.

if (frameCount % 2 == 0){   
  background(z, x, c);
}else{
  background(c, x, z);
}

Warning: If you're on a high frame rate this will essentially look strobey / flashy.

Given the r,g,b values are random anyway, perceptually it will be hard to tell the difference.

(It's unclear what the overall effect you're trying to achieve is, visually, but at least you have a hint above on how you'd swap between two colours)

Assuming you have a boolean variable called initialized, with its initial value being false, then you can check for its value and set it to true if it was false. This will make sure your variables are initialized at first call, but will not change their value afterwards:

void draw() {
    if (!initialized) {
        z = float(random(255));
        x = float(random(255));
        c = float(random(255));
        initialized = true;
    }
    background(z, x, c);
    delay(500);
    background(c, x, z);
}

If you slow your demo way down it is easier to see what is happening. You can't change the background color in a single frame; you only get one shot/frame. But what you can do is set the background color only on the even frame counts and then use the swapped values for the odd frame counts. The console output confirms that the middle value stays the same while value 1 and value 3 swap places for the alternate background color.

float z, x, c;

void setup() {
  size(200, 200);
  background(255);
  frameRate(1);
}

void draw() {
  println("===============================");
  if (frameCount % 2 == 0) {
    z = random(255);
    x = random(255);
    c = random(255);
    println("1:", z, x, c);
    background(z, x, c);
    delay(5000);
  } else {
    println("2:", c, x, z);
    background(c, x, z);
    delay(5000);
  }  
}

console output:

Addendum re: original post

I think the premise that the rgb values are changing during the life of a single frame is incorrect. The following slowed down version of the original post shows that the values do not change. In addition, the second background() call has no effect. The background color of the window does not change until a new frame is called and a new set of random r,g,b values are generated. This occurs with the first background() call only.

float z,x,c;

void setup(){
 frameRate(1); 
}

void draw() {
    z = random(255);
    x = random(255);
    c = random(255);
    println("1:", z, x, c);
    background(z, x, c);
    delay(5000);
    println("2:", c, x, z);
    background(c, x, z);
    println("====================");
    delay(5000);
}