Chromatic aberration occurs because the light of different wavelengths takes different paths through the lens that may not be in focus on the sensor. As the light is a mix of several wavelengths, the lens will focus the colors differently and create color fringes on edges of high contrast areas.
Since chromatic aberration results from colors that have shifted, white or light color on a dark background will have colors on either side. The most common effects are seen as red/cyan and blue/yellow fringes. Green/purple is one of the most unsightly, but this should not be confused with purple fringing.
Steel, chrome, and other metallic products often give rise to extreme contrast that can generate purple fringing. Purple fringing is, like chromatic aberration, an artifact that occurs because a lens interacts differently with the light of various wavelengths. Unlike chromatic aberration, purple fringing will not usually show fringes of different colors. Purple fringing is mostly visible on the edges of very high contrast image areas such as metallic products or branches on a tree against a bright sky.
Purple fringing is often seen on images that also show chromatic aberration. Wide-angle lenses are more likely to show this artifact.