The current way of styling timeline cells - is through the
timeline_cell_class(task_cell_class in old versions) template which returns the class name.
Unfortunately, there is no way to return color styles directly, so I only can suggest you define different classes to colorizing, like :
Also, you can try to implement some custom solution to change class colors directly from js if you don’t want to create many classes, unfortunately, I can’t give you any advice on implementation.
Templates of the Timeline Area :