The Slashdot Firehose where readers screen submissions has a rainbow color scale with red meaning "popular" and violet fading to black meaning "unpopular". Rainbow color schemes are a chore to read because there is no natural mapping between colors and level of popularity. One must memorize which colors mean what or refer back to the legend. An alternate color scheme such as an intensity gradient from white to grey to black naturally maps intensity to popularity; its meaning is quickly learned by readers. See the discussion at Ask Edward Tufte and the Color Brewer for thoughts and help on generating appropriate colors.