A colour monitor display output is derived from mixing three primary colours. For ‘additive mixing’ the output becomes whiter as each additional colour is combined. This is opposite to say the artist mixing paint (subtractive) tractive mixing) where the mix darkens as each colour is added. Colours are referred to as ‘Hues’. The computer display is capable of reproducing any hue within the visible spectrum by illuminating certain proportions of each colour. For example, by activating the red and green triad pixels, the eye is deceived into thinking it is seeing yellow. To make yellow appear orange then the red output is driven harder relative to the green. Increasing the intensity of all colours together increases the overall brightness of that colour. You can see the individual colour pixels by examining the colour bars on your PC screen with a magnifying glass.
A black colour bar cannot be shown unless its displayed against a contrasting background as all three outputs are turned off. White on the other hand is when all three outputs are set to maximum intensity (fully on). Greys are produced when the same level of reduced intensity is applied equally to all three outputs, red, blue and green.