Sometimes, chart authors get creative. Maybe they think their chart needs more visual interest. Or maybe they think extra colors will appeal to the audience. This can be misleading. So use color carefully.
Important: Don't use different colors for different bars unless there's a good reason to.
Color, like bar length, can provide some information about the data. If you want to highlight a trend in one category, you can color that bar and make it stand out. But don't make the mistake of thinking that color is decoration. Bar color can be read as information.
You can add visual appeal by choosing one pleasant color for the columns, but there's no need for each bar to have its own color. The category axis will describe each variable by name; you don't need to describe it with a color.
Nevertheless, when you want to point out interesting or untypical categories, go ahead and use another color. Use color like you would highlight text in a document.