In case of light, if all colours of light in the VIBGYOR spectrum are combined, we get white light. And if no light is mixed, that's black. But in case of water colours, if we mix all colours (violet, Blue, Green, Yellow, Orange, Red and some more, obviously not every one in the spectrum), we don't get anything near a white colour. Instead, we get a rough blackish mixture.
So, in a way, in case of water colours, we start with white(say if we start everything by mixing stuff into white paint), and then if we mix everything, we get black.
But in light, we start with black(no light), and if we mix everything, we get white.
Assuming the colours don't react, what's happening?