"Shy," said Lymond with simplicity, "as a dogtooth violet".
Lymond says this as he pins a dark cloak about his chin to avoid notice in the streets. "Shy as a violet" and similar idioms use the violet as a symbol of modesty.
The usual expression is simply "shy as a violet", not "shy as a dogtooth violet". As the dogtooth violet is not a true violet, Lymond may be hinting that his real reason for wanting to avoid notice is something other than modesty.