Can't draw point in white texture by OpenGL ES

[nickshen]

I create a demo which base on GLPaint. And I add a white image as background texture. But I can't see anything when I draw some points with the brush.(I can see the painting if i change the background as black).

//set point size
glPointSize(8.0 / kBrushScale);

//set the RGB color
glColor4f(1.0,0.0, 0.0, kBrushOpacity);