I have a code which basically bitshifts the alpha of the pixel array and brings it back in, essentially fading the background.  however, when I shift it, the white parts of the original background becomes blue.  is there a reverse somewhere I just don't know about? here's my code

PImage last;
float x,y;
float dx, dy;
void setup(){
 colorMode(RGB,255,255,255,100);
 size(400,400);
 last = new PImage(width,height);
 x=50;
 y=50;
 for(int i=0; i<last.pixels.length; i++){
   last.pixels[i]=color(0);
 }
 dx =350;
 dy=50;
}
void draw(){
 loadPixels();
 arraycopy(last.pixels, pixels);
 updatePixels();

 //draw beneath here if you want to keep

 noStroke();
 fill(0,125,255);
 ellipse(x,y,5,5);
 x++;
 y++;

 //draw above here if you want to keep

 loadPixels();
 for(int i=0; i<pixels.length;i++){

   int a = 10;
   a=a << 24;
   int b=pixels[i] & 0xFF;
   int r=pixels[i] >> 16 & 0xFF;
   int g=pixels[i] >> 8 & 0xFF;


   last.pixels[i]=a | r | g | b ;

   /* float r = red(pixels[i]);
    float g = green(pixels[i]);
    float b = blue(pixels[i]);
    last.pixels[i]=color(r,g,b,10);*/
   //last.pixels[i]=pixels[i];
 }

 //draw below here if you want to refresh every frame

 noStroke();
 fill(255,0,0);
 ellipse(dx,dy,5,5);
 dx--;
 dy++;

}


the white ball should leave a white streak but the red should slowly fade out.

.... any help would be appreciated

Or since you're only modifying the alpha value, not the rgb values, you could further simplify your inner loop as:

   int a = 10; // any value in 0..255
   last.pixels[i]= (a<<24) | (pixels[i]&0xFFFFFF);

nice indeed