Just like you can use Black, White, and Grey, without a hue
But you can still combine some Red and Yellow to make Orange, then mix in a spot of Black to make Brown… Or add a dash of White to Red, for a Pink.
(Or perhaps it's better to look at Yin and Yang as making up the texture of a base "emulsion" that the Elemental colours are added to, like mixing paint? And then you can add glitter or glow-in-the-dark pigment if you want to represent Natural Energy or "Biju-juice"…)
Actually, I think I've just come up with a better analogy: baking! Yang, being the solid physical energy, is your dry ingredients — stuff like flour and butter. Yin, being the flowing mental energy, is your wet ingredients — water and oil. You can use both sets independently, but for the best results you normally want a mixture. More Dry than Wet results in a solid dough, with which to make crunchy biscuits. More Wet than Dry results in a soup or a gravy. Somewhere in between lies batter, for making nice fluffy cakes.
Then you add some chocolate, or a dash of vanilla essence, perhaps some coffee or some cinnamon, which represent the Elements!
