Add tooltip next to Epic colour widget explaining how it will be used

The following comment from !90144 (merged) should be addressed:

Also I notice that while we've added Color dropdown in create page as well in the sidebar, we don't provide much of a context regarding the usage of the set color, so what if we put a little ❔ next to Color header of the dropdown and show a tooltip or a popover there stating that Selected color is applied to timeline bar for this epic in the roadmap? As for someone who isn't using roadmaps as much, it may be a bit confusing to understand the intent of this feature, wdyt?