Most brands have a signature colour, or colours. There are the single statement colours of red for Coca-Cola, or magenta for T-Mobile, or turquoise for Twitter and the two-colour approach of IKEA, McDonald's or BP. And even some multi-colour brands, of which Google springs to mind.
Some colours are used less than others, especially as single branding shades - brown is a case in point, with UPS the notable exception in the use of this colour. And a colour that's a rare tone in nature is also relatively scarce on the branding front: purple.
Purple, as a non-primary colour, has a complex set of associations. The connotations of luxury, wealth and nobility may have influenced its use for indulgent brands such as Cadbury's or Milka, or Silk Cut. The links with creativity could have been grounds for Yahoo! changing their logo colour. But purple also has more difficult connections - with spirituality, magic and even emotional immaturity according to some - which makes it a tricky choice for a brand's uniform.
However, if one colour was in evidence in the UK in the last couple of weeks on top of all the red, white and blue, it was purple. But maybe this was a lucky choice in this case as purple goes so well with gold.