Describe the feature:
Include the transparency option on the color controls for "series color" on bars / lines / area (different from existing fill)

This feature exists on heatmap and table color by value

Describe a specific use case for the feature:
When I am including a series and its time shifted counterpart
I need to reduce the opacity of the shifted series using the same color
So I can visually see that it's a "shade" of the past

CC @elastic/datavis as this is most definitely a datavis design request / discussion