Is it possible to Set Field Value of a Main Line field if a Sublist field contains a specific value.

Yes it is, see attached

I think they want to use the line value as a criteria, not a value, which you still may be able to but Im pretty sure it's not possible or at the very list is very limited currently

Correct, I was hoping to the use the line value as a criteria. Noted, and thank you!

oh, like if line field value contains "blue" then set field value in main line field "XX"