how to assign different colors to different bars

Depending on the dimension you have used in the report, you can use the same report to the color shelf and it will assign the different colors to different bars. Later if you want to change those colors, just follow the legend and select the desired color.

For example, if I have a report for segment vs sales where the segment is dimension. I have 3 bars for 3 segments and so, if I will add segment to color shelf also, it will create all three bars different colors as shown below-