There are a few ways to do this. Almost none of them is something a "designer" should take on. All of them require skill and understanding of marketing, not design, so if you want to do this, make sure you have marketing responsibilities, not design responsibilities. In other words, if you are a designer who wants to make the logo a different color for fun, for a particular brochure, you have the wrong level of authority. If you are the divisional manager in charge of positioning the "boutique division" strategically in the marketplace, and changing the logo color is an idea that popped out of your marketing strategy, THEN you can try the stuff below.

Breaking the policy for pure design reasons is both stupid and doomed to failure. Marketing enforces a brand policy for marketing reasons, not design reasons, so frame the conversation at the right level to start with. Then...

1. Mutiny, keep head low: Just break the guidelines anyway, and hope you don't get found out. Lead time = 0; cost of being found out = could get you fired. If you get a huge success out of it that doubles revenue in a week before marketing can move, you could STILL get fired.
2. Get a leash: Find out who is in charge of enforcing marketing doctrine in your organization, and just ask for permission to bend the rules a bit. You can make the argument on two levels: the exposure isn't critical OR the cost of enforcement is too high given what you want to do (no point invoking a $5000 full-blown brand audit process for a $50 expense that will be seen by only 10 outsiders)
3. Make the case and accept the verdict: Use marketing theory (branding, positioning, common knowledge about when to create a sub-brand vs. new brand...). This will work if a) you actually understand the current marketing strategy in all its glory, well enough to present a reasoned case b) marketing is willing to engage in the conversation. The marketing department will almost certainly reserve the right to make the final call even if they agree to the conversation (which they may not)
4. Fight: If you try 3 and don't like the verdict, you COULD escalate and try to start a conversation at a higher level...not recommended unless it really IS super-critical and you have the air-cover to fight the fight. If you try this, you are essentially trying to argue for a change in the fundamental business model of the company. Obviously no design level driver can sustain this level of conversation. This needs a full-blown business case for a different marketing policy that gives your boutique division the room it thinks it needs.