Is there any principle for the Gantt chart's bar color? Can you change it?

Answer

It only matters of what situation they were into. If they were in parent-children relationships the color changes from green to blue and if you have completed those issues the color change to light blue or light green according to their relation. For the addition, if you make baselines, the light gray bar will appear on Gantt chart.

You are able to change the bar colors by using JQL (Jira Query Language.) See Gantt color settings for the details.