How do you change the colour of each category within a highcharts column chart?

You can also set the color individually for each point/bar if you change the data array to be configuration objects instead of numbers.

data: [
      {y: 34.4, color: 'red'},     // this point is red
      21.8,                        // default blue
      {y: 20.1, color: '#aaff99'}, // this will be greenish
      20]                          // default blue

Example on jsfiddle