Black for professional use, or where you are exposed to the risk of theft. Chrome is just too conspicuous. I know a lot of photogs used chrome M4s etc over the ages, but that was from a time when a lot of very cheap cameras were chrome. Now it seems a genuinely metal, chrome all over camera is quite rare. And there is nothing cheap or old looking about a Chrome MP with chrome lenses!
Although it would be nice to think that people look at chrome cameras and think they are all old, I believe magpie syndrome affects chrome cameras, especially with chrome lenses. People see something shiny and think it is valuable.
For example, an Afghan walked up to me while I was taking photos and said of my chrome MP, 'That is a very nice expensive camera.' Nobody ever bothered even to look at the black MP I used exclusively over the preceding three years.
I am sure a beaten M3 does not look expensive, but as a friend commented on the MP, 'it has very clean lines and so does not look like an old Soviet camera.' He knows nothing about cameras, but it makes sense.
I am seriously thinking about getting the chrome MP painted black. I have seen so few BP 0.58s about and would have had one if I could, however I needed one to shoot and so took chrome. CRR Luton do an off gloss paint job that would be nice and inconspicuous. In sunny environments, liek Afghanistan, chroe glints and reflects light. Thats not a good thing. Movement of the camera becomes visible and attracts attention.
There is another advnatage to having all cameras the same colour - people don't know how many you have in your cameras bag....