Madonna in Malawi, again, visiting the Home of Hope orphanage; Scarlett Johansson in India on an “awareness-raising” visit for Oxfam; Angelina Jolie in . . . um . . . I can’t keep up – somewhere hot and dusty and grindingly poor. You could say the charity mission has now become an official part of the celebrity circuit: Cannes (for the film festival), Los Angeles (for the Oscars/MTV awards), the Maldives (for a photo-opportunity holiday) and now a Visit to Somewhere in the Third World to Highlight the Plight of Those Less Fortunate.

So far, so saintly. But nothing in celebrity-world is straightforward. When it comes to the famous, we are highly suspicious of their motives, which may be anywhere from deeply cynical (“She is getting so much press with that goodwill-ambassador stuff. Get me a Third World cause now”) to patronising.

However well meaning, modern celebrities are required to behave impeccably in the field, or risk our sniping and ridicule. This means not visiting refugee camps with just-blow-dried hair, and not insisting on having your food jetted in from Spago. But the big clue as to whether or not a celeb is bona fide, or just a Bono wannabe, is what they wear and how they wear it.

Dressing in the charity zone is probably the biggest challenge the modern celebrity faces, and not just because they can’t take hair, make-up and three kinds of therapist. We read a lot into the wardrobe choices of our stars in the field – more than their Oscar-night dresses, jogging outfits or even their post-break-up party frocks – because there is no rule book, and so many ways that they can get it wrong. This is the ultimate test of their sensitivity and, arguably, the nearest we can get to finding out what sort of people they really are.

Take Madonna. On her return trip to the Home of Hope orphanage in Malawi, she wore camouflage combats and heavy-duty lace-up boots. Then, when she fancied a change, she knotted a kikoi (African sarong) over the trousers and swapped her plain T-shirt for one with a rock’n’roll logo. This is a good case in point, because her look appears to confirm several of our worst suspicions, namely:

a) She thinks she is in a war zone. All celebrities fancy themselves in combat gear, especially the jungle-warfare look. It’s MASH, it’s Hanoi Jane, it’s Martin Sheen in Apocalypse Now. Whatever Madonna was thinking, the military look is not appropriate in this context. Given the rumours of her ambitions to take over the orphanage, it seems ill advised to dress as if she is in someone’s army.

b) She thinks she is down with the natives. There is always the lurking suspicion that people such as Madonna are really interested in places only for their materials and visual references. Wrapping yourself in a kikoi looks too much like she’s just issued the order, “Bag up any you can find in the village – we can use them on the next tour for, like, drapes.”

c) She thinks this is a rock’n’roll tour. This is an especially big risk with musicians, who are inclined to see the world as one big stadium (“Hello, Malawi”). You don’t want to look as if you think you’re cool when visiting a place where people have nothing: not only is it in bad taste, it also suggests you are gradually making the shift from playing rock god to playing God.

So, the superstar in the charity zone is in a tricky position. They must blend in, but not actually adopt the local costume. They should tone down their regular charity look (see Mariah Carey at the opening of Oprah’s South African girls’ school in plunging zebra-print cocktail dress) but not pretend they are on their gap year (Johansson’s mistake). They must not look as if they are either seeking to have a glamorous, fun time or trying to be really, really ordinary. And, although you wouldn’t have thought it needed saying, when opening your own charity school, it is not appropriate to wear diamond earrings the size of conkers. In other words, humanitarian chic is a minefield.

On the subject of minefields, there was, of course, one celebrity who knew exactly how to do humanitarian chic – and in the prestylist era. Ten years ago, Princess Diana was walking the landmine fields in Angola wearing a white shirt, khaki trousers and loafers. Her hair was done, but not too done; her jewellery and make-up were minimal. No sunglasses, the better to connect with the people on the ground. No nail varnish or rings, the better to demonstrate she was prepared to get her hands dirty. This was her regular mission uniform (in Bosnia, the shirt was sometimes pink), and its purpose was to look understated and professional, but elegant, as the job description dictated.

The first rule of humanitarian chic is to look as if your own taste and preferences are subordinate to your role as goodwill ambassador or Red Cross representative. But you also need to look deserving of your role, and you want to appear sober and modest. Cleavage is probably the number-one enemy of celebrity mercy-mission dressing, closely followed by sexy, in any sense other than the totally accidental.

Which makes it all the more surprising that Jolie is probably the best-dressed mission celebrity of them all. Maybe it’s because she works a contemporary version of the Diana formula – jeans, T-shirt, baseball cap. She always has her hair pulled back, and the glasses come off, unless she is on the way to the airport (which is often). According to the UNHCR, for whom she is a goodwill ambassador: “We don’t have a dress code per se, but she wears what all the team wear, which is practical field clothing. We want them to be ambassadors when they come back, and not magnets for attention when they are in the field.” Makes it sound so easy, doesn’t it?

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