The choice of background colour is, as always, a personal taste thing. Jens is right about some coloured backgrounds could reflect light of that colour onto the model but this is not too serious unless you are submitting to a magazine for publishing as they may misinterpret the colour balance for reproduction.
What is more important is the way that different backgrounds can influence the automatic exposure or even the inbuilt light meter when using manual exposure. If you use a white background with plenty of it showing, the cameras meter will try to make the scene a neutral grey. This is not a fault, it is just the way it works. So a white background may cause the subject to be under exposed.
The opposite effect is caused by a black background as, once again, the camera will want to make it neutral grey (to be precise, it is called 18% grey) so the subject will be over exposed.
You can actually see that effect in your pictures above, the lighter background shows as a darker tank, the darker background shows a lighter tank.
The obvious solution is to use a neutral grey background so as not to confuse the camera but, with a little experimentation, you can do some test shots using manual exposure or exposure compensation to ‘correct’ the exposure. As a rough guide, using a black background, you may want to decrease the exposure 2/3 or 1 full stop below the cameras suggestion. The opposite true for a white background but experimentation will determine what is needed for either scenario.
For my ‘final reveal’ pictures, I used to use a blue background which, generally speaking, was similar in tone to a neutral grey. Now I tend to use a white background and increase the exposure to compensate. This use of a white background, properly exposed, means that the backround is pure white and, as such not distracting but can appear a tad clinical.
It also depends on what camera you are using and what control you have so if you have specific questions on my ramblings, please shout out.