I'm not sure that I caught the gist of the two color fields,
but in general you can keep a line continuous but change
the color along the way but changing the pill on the color
shelf to an Attribute.
You can also have two different fields on the color shelf
control-clicking both and dragging both to the color shelf,
or by adding the second to the detail shelf and then
clicking on the icon to its left and changing it to color.
In your case, I'm not clear on whether you need ID on
the color shelf. If you do that, you will need to go into
every date and set the color legend.
Please see workbook v2018.1 attached in the Forum Thread.
285878color.twbx 1.6 MB