|Daniel Smith watercolour sticks used to create a 19 colour palette - no pans needed.|
To take it up to 20 colours you could add Indanthrone Blue - a gorgeous rich deep blue, or rich Green Gold - a very green-yellow, or Graphite Grey - a graphite watercolour that is fascinating to use, or raw sienna - I often like to have yellow ochre and raw sienna as extras as well as my favourite goethite since the pigments PY43 and PBr7 mix differently.
A larger palette might fit 24 colours so I'd add the above colours and perhaps Quinacridone Burnt Orange as the 24th. There are 51 sticks to choose from. (You can see them here)
I have simply wet the bottom of each 1cm of watercolour stick and pressed them into the re-purposed palette, which was an early 1900s cigarette case. Provided they are used with care, and not too much water, this palette is a compact and simple one to use.
Happy painting :-)