Within the context of the keyboard only (ignoring the rest of the screen) - it (nearly) doesn't matter.
Since it's grayscale you are talking about (luminance) most colour theory goes out of the window. The contrast between white and black is exactly the same as that between black and white (or any other two grayscale colours for that matter). This would not be the case if colours were involved since the smaller the colour patch the more important luminance becomes.
If there are more than 2 grayscale colours on the keyboard, read a bit on Luminance Nonlinearity.
Days and Nights
As far as night/day differences, more white means more brightness, which means a narrower pupil. In dark settings, often once you look away from a bright display, it takes a few seconds for the pupil to adjust its width to the low-light environment. In day light, the pupil width is little determined by the display brightness, since there are plenty of photons emitted from the environment.
To support night (low-light) interaction better, a darker interface would be beneficial.
In addition, you may wish to base your decision on other colours/grayscale on screen.
Reference: Visual Thinking for Design (Ware, 2008); Chapter 4 (Colors).