"For experienced painters, a large palette offers shortcuts and conveniences. But for beginning painters, it's better to start off with a small number of pigments and to learn each of their properties and their combinations."The palette you use will typically include both opaque and transparent colors (and semi-transparent/opaque). Knowing that there's a difference is an important step. It helps explain, for instance, why some colors will hide what's underneath far more than others.
James Gurney, Color and Light page 103.
Also with color mixing, where you'll waste less paint getting to the color you want if you add a little opaque to a transparent color, rather than the other way round. A bit like a little bit of a dark color has more impact than a little bit of light. With time, this knowledge becomes instinctive. A bit like knowing how much a recipe means when it says to add "a pinch of salt".
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