In our logo, the three coloured circles join together forming Ontario's provincial flower, the trillium. But when you try to do the same, you won't see a white flower.
Drag the coloured balls below around. How do the colours change when you drag one ball onto another? What happens when all three colours overlap?
Red, green and blue are the additive primary colours. The colours you get by overlapping the balls in pairs are the subtractive primaries.
Combining all three additive primaries gives white, so leaving just one of them out is like subtracting that colour from white. This is the way light mixes (try it with coloured light bulbs).
But why are the balls on a black background?
You have to start with no colour - or no pixels turned on - so you can see what happens as you gradually add colours.