I wanted to reply BMG, but I guess it won't answer your question directly. I have a white spazzoletto Maggie, so I don't have experience of the raspberry.
My care cards says to collonil, which I do and she's looking good, but is bit prone to picking up tiny black marks - from where I haven't a clue. Don't know if it would be noticeable on raspberry.
Sorry, I can't help on prevention for possible colour loss...