" If color alone is used to convey information, people who cannot differentiate between certain colors and users with devices that have non-color or non-visual displays will not receive the information. When foreground and background colors are too close to the same hue, they may not provide sufficient contrast when viewed using monochrome displays or by people with different types of color deficits." 
"When designing a document or series of documents, content developers should strive first to identify the desired structure for their documents before thinking about how the documents will be presented to the user. Distinguishing the structure of a document from how the content is presented offers a number of advantages, including improved accessibility, manageability, and portability." 
If you use colors to differentiate levels of headings then you should also use the appropriate HTML heading elements H1 - H6.
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann