WCAG 2.0 level AA requires a contrast ratio of 4.5:1 for normal text (14 point and bold or larger) and 3:1 for large text (18 point or larger). Level AAA requires a contrast ratio of 7:1 for normal text and 4.5:1 for large text.

http://www.w3.org/TR/WCAG20/

http://webaim.org/resources/contrastchecker/