Thanks, ardneh. Apologies for the delay in responding; it's turned into a crazy week.
I see by your response that there's a detail I wasn't sufficiently clear about. My need is not to split and colour the spiral in equal segments, but rather I intend to measure out segments of differing lengths, and colour each of those segments. So the first segment might be 3 inches long, the next one 5 inches long, and the 3rd might be 1/2 inch long.
My naive hope had been to find if there's a way to simply mark out these segments manually. For instance, click on one side of the wide spiral line, then drag the cursor to the same spot on the other side of the line, and indicate that that's now a boundary between the 2 sections of the line. If such exists, I haven't found it yet, but I'm very aware that could simply be a matter of my not knowing the right terminology to search for.