"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".
Photo © Marion Boddy-Evans. Licensed to About.com, Inc.

Comments
My next work for the current project will be directly inspired by the sentence: “…a small number of pigments and to learn each of their properties and their combinations”.
Towards the end of last year I started exploring phthalo blue, putting aside my beloved Prussian blue. It’s a very powerful blue, but its staining properties can be great when you let it run and dribble, letting it mix on-canvas…!
Phtalo blue can be a transparent or opaque color regarding the brand: Transparent in Daler Rowney and Opaque in Liquitex, for instance! I just realize it is relating to the pigment composition:
- Daler Rowney PB 15
-Liquitex PB 15:3
I read that acrylic colours, most of them anyway, are intrinsically transparent. How does working with acrylics compare with oils and watercolour in that respect?
Dave, you can find many opaque colors in Liquitex, Lascaux or “Amsterdam expert” for instance. But of course don’t dilute them overly.
@Dave Wherever you read that was talking rubbish. Pigments in oils/acrylics are identical. Binder does nada.
OK, guys, thanks for clearing that up!