just a thought... you haven't got this the wrong way round, have you? If you're mixing pigments, Chillum's right. But if the question was about mixing red light, blue light and green light, the answer would be white light.
Look closely at a patch of white on your television screen. You'll see that the red, green and blue parts of the pixels are glowing equally brightly to produce the white. This is why R,G and B are called the primary colours: in mixing light all colours recognised by the eye are made up of differing portions of these three. The equivalent colours when mixing pigments (the 'artist's primaries') are Cyan, Magenta and Yellow.
simple answer is white. blue red and yellow light are made by scattering the light constituents of white light. thats why the sky is blue, the sun is yellow, apart from when it is setting or rising, as it has to pass through the thickest layer of atmosphere and scatters more.
The answer 'white' has already been given, boobesque, and now you are confusing the issue. The constituents of white light are red, green and blue - the primary colours. Yellow is a secondary colour.