Colours can be mixed as on a palette by creating a temporary frame in which you mix the colours. Tools with low intensity such as the colour laying tools, Water and possibly Smear are particularly suitable. This frame can be copied into the Clipboard and used in other documents, or exported to be used later. To use, just select colours from this "palette frame" with the colour picker.
Take into account the setting for the colour picker size, which defines how many pixels are taken into account when setting the average value.
Example of a palette frame
