Retrieve amount of red colored pixels in a set area

[Primex]

I arrived here because searching google didn't give me anything and I figured it might not be that hard to realize.

My question:
I need to create a box of coordinates x, y, width, height. Which I need to be able to search for a specific color (let´s say red) and then add up these pixel counts.

I figure it works something like this: It starts at the top left, if pixel is red (255, 0, 0) then count= count+1, next pixel. Else next pixel etc.

The background is plain white, and there is only 1 actual color used (red).

What I know how to do:
Get pixel color at specific coordinate (robot.getPixelColor(x, y))

What I probably need to know:
How to increment the coordinates (x+1 untill end, then y+1, and x+1 untill the end etc. while continuously checking for pixelcolor).

Perhaps there is a total solution, maybe it can check the entire box as a whole and give the amount of red colored pixels within it?

I know it might come across as vague but that's mostly because I have no idea how to do this and am trying to paint a picture of it from what I see as logical. Now if someone would be so kind to push me into the right direction (maybe there is a whole other solution to this) I would be grateful.

[Primex]

I think I did it, coordinates and RGB code was set for testing purposes. If anyone knows a less memory/time consuming way or just an all-round better solution, I'd love to hear it.

public static void main(String[] args) throws AWTException {

        int y = 500;
        int x = 300;
        int red;
        int count = 0;
        Robot robot = new Robot();

        for ( x = 300;  x < 901; x++ ) {

        Color color = robot.getPixelColor(x, y);
        red = color.getGreen();

            //If pixel is colored, then add 1 to count.
            if (red == 128) {
                count = count+1;
                System.out.println("Amount of colored pixels:" +count);
            }

            //If x coordinate reaches 900, then set it back to original value
            //and increment y coordinate with 1.
            if (x == 900) {
                x = 300;
                y = y + 1;
            }

            //If y coordinates reaches the set ending, then break off.
            if (y == 801) {
               break;
            }
        }
    }
}