Your response depends on the kind of person you are dealing with, and how close to reasonable the question is.
For a person who is smart, thoughtful, and has the best interests of the company at heart (your default view of everyone in the company) your best bet is to understand why they are suggesting something you clearly think is wrong. For example
Normally we wouldn't provide any links that take people off our site after going to some trouble to get them there. Can you explain how it would help us to have this link?
(The answer may be "it lets people see that a third party endorsed us" or "it will be a major convenience to our users and avoid them getting to the same place from the competitor's link." Or "oh, I didn't think of it like that, never mind.") Your formula is [Normally, usually, typically, in most cases, our original plan, the requirements we were given] [conflict stated directly.] [Ask for explanation of suggestion given that context.]
For a person who just doesn't understand timelines, planning, budgets, and other "laws of physics" of a project, and makes great suggestions (or potentially great suggestions) at the absolute worst time, you need to remind them of these laws of physics.
Neither the time till launch nor the budget makes rethinking the entire colour scheme feasible on this iteration. Other than personal preference, is there a particular aspect of this colour scheme that's really problematic for accessibility or the like?
The formula is [Remind of laws of physics.][Ask in case it's possible to do 1% of what they're saying and make them happy, or as a way to show the rest of the group it wasn't a real suggestion.][Optionally, suggest they get involved in the next iteration.]
For people with a history of blurting out ill-conceived half-baked suggestions with no idea how much work they are:
Do you have time next month, after this is live, to spend an hour or two with me going over that in more detail so we can see if it fits into the next iteration?
(They usually say no. If they accept, you can educate them during that hour or two, not now in front of everyone.)
And finally, for the "I'm so much smarter than you" guy who isn't, and who you've butted heads with before:
Thankyou. I'll keep that in mind. No promises, though.