Quick answer
Neon and Rainbow are reported appearances—not verified formulas
Current fan research supports the existence of Neon and Rainbow mutation appearances. No selected source provides dependable rates, stat effects, eligible pack tables, or Luck stacking rules.
Check the exact label and visual shown on your letter in Roblox. Treat mutation potions as update-sensitive resources, and do not infer a probability from a short opening video or a lucky session.