What's the best palette to use for mixing watercolour? Plastic, porcelain or metal?

Porcelain palettes are the best to use with watercolour because they are absolutely water impermeable, and hence don't stain and easy to clean. There's a reason why bowls, plates, washbasin and toilet bowls are made with porcelain.

Porcelain is typically made by heating kaolin to extremely high temperatures until it turns into a type of glass in this process called vitrification.

Downside to porcelain is they are heavy and can break when dropped.

When you mix watercolour on porcelain, the water tends to collect in a big pool and don't break into separate smaller pools. This allows you to spread paint out on larger surface area, and makes it easier to mix paint, and you can also see the colours you're mixing.

These are mixing wells from typical plastic watercolour palette boxes.

Notice how the paint will break into smaller pools of water or tiny beads? It's not nice to work with. The well on the right performs much better in the sense that the paint doesn't bead up much, but you can still see there's a tendency for the paint to break into separate pools. The behaviour of paint on plastic palette can be unpredictable and inconsistent as you can see above as those two wells are from the same palette.

You can actually roughen up the surface of plastic to make them more suitable for watercolour use. You can use either Magic Sponge or Magic Eraser, extremely fine grain sandpaper or sand eraser to roughen up the surface. This will allow the paint to spread out more and reduce the tendency to bread into separate pools or shrink/bead up much.

Here's a typical metal watercolour palette box. The mixing behaviour here is similar to that of plastic. So to get the best performance, it's best to roughen up the surface as well.

As you can see above, the two wells from the same palette behave differently as well. On the left, paint is able to spread without breaking into pools, and on the right there's breaking and pooling.

The downside to plastic and metal is the water beading effect. Plastic and metal will also stain so to clean them thorough, you have to really scrub them with the Magic Sponge. The advantages of plastic and metal is they are more portable since they are much lighter.

The porcelain palette that I bought above was from eBay and I got it for US $13 with shipping. You can also find them on Amazon and Jackson's ART (UK). There are many designs and sizes you can choose from.