red, green, blue rgb colors added are in sum black, but i want white color

red, green, blue rgb colors added are in sum black, but i want white color

I use the raphael jquery library to draw 3 circles(red, green, blue) with
their center on a equilateral triangle. the tree circles have an area of
intersection which is drawn black, but i want to have it white like the
most graphical programms handle the addition of the full colors red, green
and blue