From Eterna Wiki
Christmas trees are RNA puzzles, lab or challenge solutions consisting mainly or exclusively of GC Pairs, so called because they are mostly green (C) and red (G). Although these solutions typically have low free energy values for their minimum free energy structures, they are also prone to suboptimal structure formation.
Teaching about Christmas trees
RNA Lab: "Christmas Trees," "Cub Scout Projects," and "Optical Illusions" by Dimension9