Suppose you have a bar chart like this:
and it's made from a data table like this:
And you are really proud of the sales of the XP1000 and want to change the colour of its bar to red. In Apple Numbers you can't do that because the bar colour is based on the data series.
But you can fool Apple Numbers by creating two data series like this:
Then choose a Stacking Bar chart after selecting the two series of data in the data table and you'll get a chart like this:
You can change the colour of any of the series by clicking on the Fill button on the toolbar. And you can extend that beyond two series to colour the individual bars as needed.