Mythryl uses mixed-case identifiers in two contexts: to name a type and to name an API. (An API is essentially the type of a package.)
Their syntax is:
[A-Z][A-Za-z'_0-9]*[a-z][A-Za-z'_0-9]*
A mixed-case identifier should normally consist of one or more underbar-separated words:
Color
Compound_Color
Rgb_Color