Percepts have hue, brightness and saturation, correspondingly. Saturation is the dominance of a single hue. White is desaturated, and has all wavelengths equally. Colour vision is synthetic - responding to combinations or syntheses , rather than analyzing into components, like audition. Hence, vision involves colour mixing, an additive process.
White light is a clear example.
It results from adding many hues. When we mix paints we put together materials that can absorb colours, and only reflect some colours. When we add paints, more and more wavelengths are subtracted from what is reflected. The result is muddier and darker the more paints that are added. When we add coloured lights, shining on white paper, we add light wavelengths and so can produce bright, white light with many coloured bulbs.