Before you say that colors don't mix the way they do in this game in real life, consult the rgb diagram below 
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Technically, additive mixing of colours applies to light while subtractive mixing applies to materials like paint, ink, or in this case, coloured fluids, so they'd be right. However, I'm much more satisfied with the liquids having bright vibrant colours than I would be with them becoming more and more muddy-looking the more I mix them, so using additive mixing was a good design choice.