"Her cap is purple."
Translation:Jej czapka jest fioletowa.
Just to add a bit more information:
This is shades of violet, according to Wikipedia.
This is purple as defined for HTML(css).
And a few interesting quotes from Wikipedia article about purple:
In the traditional color wheel used by painters, violet and purple are both placed between red and blue. Purple occupies the space closer to red, between crimson and violet. Violet is closer to blue, and is usually less saturated than purple.
While the two colors look similar, from the point of view of optics there are important differences. Violet is a spectral color – it occupies its own place at the end of the spectrum of light first identified by Newton in 1672, and it has its own wavelength (approximately 380–420 nm) – whereas purple is a combination of two spectral colors, red and blue. There is no such thing as the "wavelength of purple light"; it only exists as a combination.
While the scientific definitions of violet and purple are clear, the cultural definitions are more varied. The color known in antiquity as Tyrian purple ranged from crimson to a deep bluish-purple, depending upon how it was made. In France, purple is defined as "a dark red, inclined toward violet." The color called purple by the French, pourpre, contains more red and half the amount of blue of the color called purple in the United States and the U.K. In German, this color is sometimes called Purpurrot ("purple-red") to avoid confusion.
In color theory, a "purple" is defined as any non-spectral color between violet and red (excluding violet and red themselves). The spectral colors violet and indigo are not purples according to color theory but they are purples according to common English usage since they are between red and blue.
After this fact finding, my opinion is, both should be accepted because it's not exactly clear what is meant by either purple or violet. ;)