Guideline 2

It should be noted that not everyone can see in all the colors, if colors aren't formated correctly it can be very difficult for blind or visually impaired users. Also this guideline can be applied to use appealing and/or eye pleasing colors.

2.1 Ensure that all information conveyed with color is also available without color, for example from context or markup.
2.2 Ensure that foreground and background color combinations provide sufficient contrast when viewed by someone having color deficits or when viewed on a black and white screen.

We can't all see like her!