Formatting a fixed curve on graph (colour, type, fill) with an expression

Hi all, I'm trying to add various granular specification envelopes to a grain size graph. However, I now want to see multiple specs on one graph, and I need them to have a different colour/fill. Here's my workflow:

- user specifies granular specs in User Report Variables (report var.gran spec1, spec2, etc.)

- graph pulls the user report variables and calls those curves from the library

Problem is, they are all the same colour! How can I write a "fixed curve expression" that auto-formats each successive spec with line colour/style/fill?

Thanks!