Obtain Color From Borders

I'm doing this project where I need to make a persian carpet using recursive methods. I already have my recursive what I'm trying to implement is the use of color. My method BorderColor draws a square using a random color choose from an array.

public void Cuadrado(Graphics g){
Random color = new Random();
g.setColor(colors[color.nextInt(colors.length)]);
g.drawLine(left, top, right, top);
g.drawLine(left, top, left, bottom);

[Code] ....

What I'm trying to do is getting the color from 4 different corners in order to create a new one and make a recursive call. I already tried using the method getPixelColor from the Robot() object but I quit it because is a recursive method and it may cause trouble with the recursive stack. I created this method to create the new color and then call it in the PersianRug method, but I dont have a clue on how to obtain and implement the new colors.

public int newColor(int c,int c1,int c2,int c3, int a){
int c4=((c+c1+c2+c3)%13)*a;
return c4;
}