Short answer: you don't tell people "the whole truth" as you see it. You definitely leave out things that are confidential, and things that are pure personal speculation with no particular grounding in evidence. Just because someone asks a question doesn't mean you must say everything in your brain ;-)
Depending on the company, you might also leave out things that are true and relevant and public, but that require some care in order to avoid the bad things eclipsing the good things. This is PR/spin/sales/presentation/tactful/sneaky, but whatever you call it you will in most jobs have to do it to some extent some of the time. So find out from your company what you need to do, then either do it or refuse to do it as conscience dictates.
If you're routinely being asked a question that you're unsure how to answer, then you should consult with whoever has authority over your interviewing activities, how it should be answered.
When you're interviewing, you represent the company. The company has an official position as to what its prospects are, and what challenges/threats it faces. This is what you present when you're representing the company, and you should find out how far you can go in honestly presenting the downsides without straying away from what the company is happy to be discussed publicly and into embarrassing or confidential information.
If the company truly is relentlessly optimistic, it doesn't admit any potential challenges to its progress or threats to its business model, then frankly just repeat that to the candidates. The smart ones will realise for themselves your company is dumb, the dumb ones will fit right in ;-) More likely you can talk about some of the downsides, you just don't know yet which ones and in what ways.
I have some premonition about the direction the company is going
An interviewee isn't really entitled to ask for or receive your personal opinion. Things are friendlier when there's a polite fiction that they do get that, but if it comes down to a choice between removing the words "in my personal opinion" from what you say, or discussing embarrassing/confidential company information with an interviewee, then in a case of any doubt whatsoever you should remove the words.
If you think that the company is growing too fast, or that it's pursuing customer X when it should focus on Y, or some other mistake that will end in doom, then that's your personal opinion. Despite this opinion, senior management makes those decisions and you go along with them for as long as you work for the company. There may or may not be an appropriate forum within the company for you to air your alternative strategy, but "at every opportunity including interviews" certainly isn't it ;-)
If you feel that the company's official position is unrealistic to the point that they're asking you to lie, then again you need to speak to people within the company to work out in what sense the thing you're expected to say, is true. If they can't convince you that the company is basically honest (albeit not forthcoming with bad news), then you have a problem with the company, not just with interviewing. It would be the same in any situation where you're in effect being asked to comment on the company using your inside knowledge. If really pressed, the short answer is that you won't give a personal assessment. You could point out that while you won't disclose your personal assessment, it's sufficiently positive that you choose to work for the company! Of course, if you can't represent the company without (as you see it) lying, then that ultimately might limit what roles you can take within the company. At the extreme, either the company can't let you speak to outsiders or it can't let you work for it at all, so it's really quite important to make absolutely sure of any doubts you have in what you're saying.
Note that there may be things that would put off a specific candidate, but aren't generally embarrassing to the company. For example if a start-up company is expecting to exit via acquisition in 2-5 years, and a particular candidate has been treated horribly in previous acquisitions, then that might put them off even though the company's prospects are objectively excellent. You need to be absolutely certain of your role in the hiring process before you get into this stuff with a candidate. It might be someone else's job to address candidates' concerns (in this example: to go through with them what would likely happen in an acquisition), which they don't get a chance to do if the "bad" news comes from you.
Finally, be aware that there's a completely alternative approach, which is that it's perfectly fine to lie and that you should do so. Shockingly, not everyone in the business world is honest, and sometimes not even everyone on your side is honest. It sounds like you're not happy with that, but be prepared to deal with people in your company who do feel that way (or anyway who appear to feel that way, because what they're saying flat contradicts your assessment of the situation). And deal with them without just outright calling them liars and starting a massive fight that won't end well for anyone.