Comment by LeonM

These CSS colors consist of 3 bytes in hex, the first gives the red value, second green and the last is blue.

So:

  #FF0000 = 0xFF, 0x00, 0x00, which is pure red
  #0000FF = 0x00, 0x00, 0xFF, which is pure blue
  #FF00FF = 0xFF, 0x00, 0xFF, which is red and blue mixed, so it's purple (violet)
  #FFFFFF = 0xFF, 0xFF, 0xFF, all colors combined is white

etc

#C0FFEE = 0xC0, 0xFF, 0xEE, so by 'parsing' it in your head yo can see is mostly green and blue, combined that's cyan.