"White" is not a color, because color implies single frequency electromagnetic wave in visible spectrum. Look up that on wikipedia. "Pure" white light would contain all frequencies in visible spectrum with equal level, but light that is formed to match human eye response might be perceived as more "pure" than if all frequencies are flat. So, you could say that white light is in the eye of observer. You could also trick human eye by having spectrum of just 3 frequencies to perceive it as complete spectrum, which is used in all monitors, printing, white LEDs,...
So if you would like to sense printed paper you have to sense ratio between colors used and decide what ratio is "white".
There are quite a few companies that produce color sensors in various forms and specifications, so here are few.
www.taosinc.com **broken link removed** **broken link removed**
They would also likely include some customer education publications in form of application notes, read them! You might not need to go in much depth for your application, but it will be easier when you understand this in more detail. Again, wikipedia has very detailed pages on this subject.
Here is an example of an interesting project build around PICAXE-08M and colour sensor TCS230: https://www.rev-ed.co.uk/docs/axe045.pdf
Based on this example you should be able to build a white colour sensor ..
I am serious.
I guess the working environment may be dark, bright. It may affect the result of
the color sensor, so super white LED is used around color sensor to make sure enough light