size(200, 200);
background(255);
noFill();
stroke(0);

int x1 = 0;
int x2 = width;
int y1 = 0;
int y2 = height;
int lineNum = 40;
int lineGap = height/lineNum;
int clrGap = 256/lineNum;
int clr = 0;
int counter = 0;
while (counter<40){
stroke(clr,0,0);
line(x1,y1,x2,y2);
y2 = y2-lineGap;
clr = clr + clrGap;
counter = counter +1;
}