How to visually encode something "real/actual" vs "ideal/expected"?

In a data visualization, I need to show two lines: one representing an actual, observed value (in the example below: red), and one representing a norm, or what this value should be in an ideal case (in the example below: green).

While it feels very intuitive to encode "good" values as green, and "bad" values as red (and similar conventions exist for representing magnitude, terrain height, and many others), I'm struggling with coming up with an intuitive visual code for "real" vs "ideal".

Is there any conventional, intuitive way to do that? Or is it completely arbitrary?

Answers may include not only colors, but also styling the lines in any other way (make them wider, dashed, and so on), or even representing those values in some other way (not as lines), as long as it's applicable to a graph like the one below.

Disclaimer: Obviously, I will also add a legend. My question is only about the visual style.