The phrase "most heliotrope" can be used to describe a vibrant shade of purple. However, if you are looking for synonyms for this phrase, you could use terms such as "deep lavender," "vivid violet," "rich mauve," or "intense magenta." These words can help to convey the same idea of a bold, eye-catching shade of purple that stands out. Additionally, other synonyms that might work could include "amethyst," "periwinkle," or "lilac." It all depends on the specific shade of purple you are trying to describe and the context in which you are using the term.