I think I know what you're talking about, and it probably doesn't have anything to do with Wikipedia's font.
Most modern operating systems have some form of text antialiasing: instead of seeing letters and symbols in blocky outline, they are rendered with various degrees of shading. This means that, for black text on a white background, some pixels will not be displaying black, but rather some shade of grey.
Now, your computer screen is probably not perfect: in fact, many contemporary screens are not only of sub-par quality, but also miscalibrated. Since each pixel is composed of red, blue, and green components to display any colour through additive colouring, what's probably the case is that the miscalibration is causing less green to be displayed than should be the case (or more green elsewhere). As a result, you see (or think you see) purple (technically, magenta) on the edges of the black text, where the antialiasing causes some "grey" to be present.
You might be able to solve it by a) adjusting your monitor's settings, b) getting a new monitor, or c) telling your internet browser to override Wikipedia's font with one that is not antialiased (this would make the font ugly, though). You are less likely to see this type of problem on a Mac—their monitors are generally calibrated decently out of the box.