Here's the steps on how to change the color of the gantt bars in your gantt chart. Just double click on a task and you will see the option called "color by" in the task details view under the "general" tab.
Then click on the little pencil icon and this popup will appear, the default setting will be that the gantt bars are colored based on the color assigned to resources assigned to tasks. However you can change this by selecting the "custom" option and then selecting a color of your choosing and hitting "save" then "save task".
Let me know if this was helpful or if you have further questions.
Kind Regards, Sophia