As soon as you complete Konley's Ruby Vibrations questline, he asks you to help him with this brief quest. Magnus has a mission (Making Light Part 2) that requires you to have 2 of the Rainbow Essence as well, so if you can time it so that these two missions fall together (or save the essences until you complete both), you can knock out two quest requirements at once.
Once you complete this quest, Konley will resume the gemstone theme, with his Diamond Harmonics questline.
Have 1 bed of Rainbow Flowers
Gather 4 Rainbow Essence ()
Make 1 Prismatic Crystal Ball