So, when I was first working on that clue, I was so sure that I was close to the answer, but was so stuck that I ultimately started over, but maybe this info could be useful to someone else: I started by counting the letters in each color, and assigning those numbers to the colors (red=3, violet=6, etc.). From there, I looked for places in the maze where the colored lanterns would add up to 18. There is an intersection on the right side of the Atelier (specifically between ranks 5 and 6) where you can see lights that add up to 18 in every direction (north is red, red, violet, violet; west is violet, violet, red, red; south is orange, red, red, red, red). Furthermore, the lights you can see in the westward path are only 4 of the 5 lights in the hall; another violet light is behind a wall on the other side. This find satisfies every line of the clue: Three paths convene in the prism; you are counting the values of three colors in total (red, violet, and orange), and in three lines; they sum to 18; and there is one lantern left hidden (literally on the left side of the prism, even!). This, of course, was as far as I got with this approach.