gistfile1.js

 function start(){

        // save the context state
        ctx.save();

        // draw the overlay
        ctx.drawImage(overlay,150,35);

        // change composite mode to source-in
        // any new drawing will only overwrite existing pixels
        ctx.globalCompositeOperation="source-in";

        // draw a purple rectangle the size of the canvas
        // Only the overlay will become purple
        ctx.fillStyle=newColor;
        ctx.fillRect(0,0,canvas.width,canvas.height);

        // change the composite mode to destination-atop
        // any new drawing will not overwrite any existing pixels
        ctx.globalCompositeOperation="destination-atop";

        // draw the full logo
        // This will NOT overwrite any existing purple overlay pixels
        ctx.drawImage(logo,150,35);

        // draw the truck
        // This will NOT replace any existing pixels
        // The purple overlay will not be overwritten
        // The blue logo will not be overwritten
        ctx.drawImage(truck,0,0);

        // restore the context to it's original state
        ctx.restore();

    }