Which colour is this?

Purple is any of a variety of colors with hue between red and blue. In the RYB color model historically used by painters, purples are created with a combination of red and blue pigments.

Purple has long been associated with royalty, originally because Tyrian purple dye, made from crushed shells, was extremely expensive in antiquity. Purple was the color worn by Roman magistrates; it became the imperial color worn by the rulers of the Byzantine Empire and the Holy Roman Empire, and later by Roman Catholic bishops. Similarly in Japan, the color is traditionally associated with the emperor and aristocracy.

Purple is closely associated with violet. In common usage, both refer to a variety of colors between blue and red in hue. Historically, purple has tended to be used for redder hues and violet for bluer hues. In optics, violet is a spectral color: It refers to the color of any different single wavelength of light on the short wavelength end of the visible spectrum, between approximately 380 and 450 nanometers, whereas purple is the color of various combinations of red, blue, and violet light, some of which humans perceive as similar to violet.