If you have more than one slice created for a configuration, Structure.Gantt will apply slices in the order they appear, from top to bottom.
Once a slice has been applied to an issue, no other slice can affect it. If you need to apply a different slice first, move it up within the list.
In the example above, Slice 1 makes all epics appear purple in your chart. If Slice 2 specifies that all issues from a specific project should be orange, the epics from that project would still be purple. If you want the epics from that project to appear orange too, you would need to place Slice 2 above Slice 1.
To move a slice up or down within your list, use the Drag Bar next to the slice's name.