Issue:   How to change the colors of one or more event categories in the timeline chart.

First, you need to have be in Timeline Chart tab.  You should see a Theme tab to the right.  If you don't, click on the View menu.  Put a check mark beside Themes. 

Next, select an event.

Solution 1:   Use Apply to Category on a Theme color.

Below the list of different themes you  will use this area to make your changes.  Look below the tab labelled "Event".  There you will  see some color examples for event colors. 

Right click on any event color you like and choose  Apply to Category. Select your Category. 

The selected event will have that category will be changed to that color. If all events are selected, do the same steps to change all events in the category.

Repeat as necessary.

Solution 2: Change an Event color to something else.

Click on the event  in your chart. Above a the Event Format menu will appear.

Make your changes to the selected event, color or othewise.

Select all events you wish to look the same.

Find your new selection in the Themes are, below all the other colors in the list.  

Right click on it and choose Apply to Selected.

Optional:  Use another Theme's color.

Find another color or style by clicking through the Theme List, 

Right click on it and use Apply to Selected or Apply to Category.