If you want to clear the screen to a specific color, you should set the color of the scene. That controls the screen clearing color for the GPU.
If you just want to clear the CCDrawNode's background, you should really consider a CCNodeColor instead. The pixel shader used by CCDrawNode is several times more expensive than the simple one that CCNodeColor uses. Even if you don't have performance problems right now, it will waste a lot of battery power to draw every pixel on the screen with the extra processing needed to apply the anti-aliasing.