public void renderTiles(Graphics g, int xScroll, int yScroll, int width, int height){
      for(int x = -1; x < width; x++){
         for(int y = -1; y < height; y++){
            int xt = (x+xScroll/tileSize)%size;
            int yt = (y+yScroll/tileSize)%size;
            if(xt < 0)xt += size;
            if(yt < 0)yt += size;
            g.setColor(new Color(tiles[xt+yt*size]));
            g.fillRect(x*tileSize-xScroll%tileSize, y*tileSize-yScroll%tileSize, tileSize, tileSize);
         }
      }
   }

//pack a small opaque white area into your sprite sheet, 
//e.g. 1x1 white pixel with nearest filtering 
//or a larger area with bilinear filtering

//get the sub-image of the white pixel
Image whitePixel = spriteSheet.getSubImage(...);

...

//now in your render loop, draw many rectangles like so:
spriteSheet.startUse();

//... draw your other sprites from this sheet using drawEmbedded ... //


//now bind the color you want your filled rectangles to be, e.g. black or whatever
rectColor.bind();

for ( each filled rectangle ) {
    //draw the filled rectangle
    whitePixel.drawEmbedded(x, y, width, height);
}

//end the sprite sheet
spriteSheet.endUse();