Hex color string validation.
Splits out the hex-specific validation from Pote.Validator so that
the validator remains a thin façade dispatching to per-format modules.
Not part of the public API — used only by Pote.Validator.
Accepts:
RRGGBB(6 hex chars)#RRGGBB(with optional#prefix)
Summary
Functions
Returns the error message for :invalid_hex.
Validates a hex color code (with or without # prefix).