Meh.

  var colours = coordinates.stream().map(this::computeColour).collect(toList());

  var colours = coordinates.stream().parallel().map(this::computeColour).collect(toList());

Java doesn't do as much (well, anything) to help you make sure the implementation of computeColour is suitable for parallelisation, but that comes with the territory. On the other hand, it will automatically split the stream up appropriately for the number of cores you have.