Additive colour is the colour effects of light directly from a light source. It starts with black and mixes red, green and blue light together to produce other colors.
RGB (Red, Green, Blue) Colour is another way of referring to additive colour. The most common example of additive colour process is colour a television screen or computer monitor.
RGB are the 3 primary colours and additive colour process mixes various amounts of red, green and blue light to produce other colours. Combining one of these additive primary colors with another produces the additive secondary colors cyan, magenta, yellow. The additive colour model image shown above is a good example of how additive colour is processed.
Where blue and green are combined the color cyan is produced
Where blue and red are combined the color magenta is produced
Where red and green are combined the color yellow is produced.
When the primary additive colours are combined (red, green and blue) produce what we perceive as white light.