Below is a possible workaround example for changing the highlight color using a parameter and calculation.

1. Create a parameter "Select a Sub-Category" to trigger the parameter action.
2. Create a calculation field "Color the Sub-Category"

IF [Select a Sub-Category]="" then "Blue"
ELSEIF [Select a Sub-Category]= [Sub-Category]
then "Orange"
else "Gray"
END

3. Create a calculation field "1", convert it to "Dimension". *This is to disable the trick to disable the default highlighting. 
4. "Worksheet" > "Actions" to add a parameter action like the following.

5. Drag "Color the Sub-Category" to "Color"
6. Drag "1" to "Detail"
7. Change the Highlight to "1" like the following to disable the original effect of the highlight behavior

8. Click a line and set the colors for the selected line and the other lines.
9. Right click the parameter "Select a Sub-Category" and select "Show Parameter"
*Manually delete the contents in the parameter legend will trigger the line graph to go back to the original.

Note: Follow step 8 above to change the colors. Calculated field is for setting the Legend/text and modifying the values will not change the colors.