Making widget icons work - how to prepare them, so that setcolor will change their color?

The trick is not the size, so much as how to import the image using the image converter.

Generally I:

• Invert the image in a paint program so it's white
  
• Rescale it in a paint program so it's maybe 20x20px (widget bar is 24px)
  
• Use the image converter as 1 bit, or 2 bit greyscale
  
  

Using 1 or 2 bit means Espruino will then draw it using the current color.

There's also a 'beta' image converter with a few more features at https://espruino.github.io/EspruinoWebTools/examples/imageconverter.html

One of those is 'invert' - so you can skip the external inversion step.