for the goo mutation step, a graph has been generated that links all the colours together in a somewhat sensible way, and it steps through that graph to whichever neighbour is closest to the target than the current colour is; the random mutation then kicks in to sometimes also step it 1 neighbour in any direction.
the graph is generated by starting from each of the 255 colours, and evaluating for all the other 254 colours which of the 254 remaining colours in closest to the current colour, whilst also being closer to that other colour than the current colour is, and adding a link between those colours to the graph. Basically ensuring that no matter which colour it is morphing towards in the goo mutation (aka the 255 gleam colours) there is always a step you can take in the graph that brings you closer to the target than the current colour is and that those steps are somewhat minimal in distance.
Distance here, is measured using the CIEDE2000 colour distance metric.
Colour mixing is a linear combination of the colours in the Lab color space, and then the result is mapped to the closest in the palette, again with the CIEDE2000 colour distance metric.