Typogrophy and Color Contrast

Not only is the contrast important also the chosen typeface will make the difference in a good or bad sign. When using too bold weighted typefaces the text will look like its expanding of the sign, when using too light weighted typefaces the text will fall back into its background. Medium or Regular weights are usually the best options to choose for a good and readable sign.

--From a very interesting read

http://designworkplan.com/design/signage-and-color-contrast.html