Opposite of the word dark

The additive opposite is green (as seen on a color wheel).

The complementary secondary color is cyan.

In a matter of light, the primary colors are Red Green Blue, or Magenta Cyan Yellow. However, when you're mixing paint, the primary colors are Yellow Blue and Red. To get black, you would add the opposite, or green (actually closer to aquamarine).

In the RGB system, 255 green and 255 blue is the opposite of red.
on the colour wheel, its opposites are yellow and blue.