Why does the validator complain You have no background-color with your color?

If you don't specify color and background-color at the same level of specificity, your style sheet might clash with user style sheets. To avoid this, specify always both of them, see Section 9.1 "Color Contrast" in the W3C Note "CSS Techniques for Web Content Accessibility Guidelines 1.0" or the article CSS Color Issues by the CSS Pointers Group for a discussion.