How to split a canvas into paintable segments in Processing?

Question

Java

I have been studying the Processing environment for half a year, but recently I stalled on one task: to write a program that divides the canvas into equal cells, on which you can draw.
Below is a program that "pixel by pixel" determines where the mouse is.

int r = 25; // Размер пикселя, на который будем делить холст

void setup() {
  size(251,251);
  noStroke();
  cursor(CROSS);
}

void draw() {
  background(255);
  for(int x = 0; x<width; x+=r) {       // Для каждого r(эр)-ного х...
    for(int y = 0; y<height; y+=r) {    // ...рисуем столбец квадратов
      if((mouseX>x)&&(mouseX<x+r)&&(mouseY>y)&&(mouseY<y+r)) // Проверка на то, находится ли мышь внутри квадрата
      { fill(255,0,0); } // И, в соответствии с результатом выше, закрашиваем квадраты в красный...
      else  { fill(255); } // ...или белый
      rect(x,y,r,r);
    }
  }
}

I want to do almost the same thing in the program for which I created the question, but in it the "pixels" will be painted over only by clicking on them, and their color will be "permanent".
I will be glad both direct hints on the code, and literature on this topic.