Even if removing the color by code:
mycanvas.clearColor(d[1],d[2],d[3],2.0);
mycanvas.clear(can.COLOR_BUFFER_BIT );
The screen gets cleared at beginning of next draw cycle.
To create WebGLRenderingContext, previous drawing buffer can be preserved.
gl = someCanvas.getContext("webgl", { preserveDrawingBuffer: true });
The default is preserveDrawingBuffer: false by making this property true, previous drawing can be easily preserved