Porcelain/ceramic is the best surface for mixing watercolours. The matter is that there are no portable watercolour boxes with porcelain/ceramic mixing spaces available on the market.
They could be made as porcelain in particular can be manufactured to become in so extremely thin layers, up to the point to look transparent under direct light, but no one bothers to manufacture such boxes for outdoor painting.
The technology exists... the boxes are those that don't exist.. lol