The MSDN article that describes the use of the addHighlight method for the VWA.Shape object (http://msdn.microsoft.com/en-us/library/ff394531.aspx) states that the color parameter must specify a color using the “#RRGGBB” hexidecimal color format, for example…
nextShape.addHighlight(4, "#FF0000");
I wanted to point out that the color parameter will also accept Color Names as described in the HTML and CSS standards.
nextShape.addHighlight(4, "Tomato");
Here is a handy link to an article that outlines the standard Color Names along with a visual representation of each (even the hex value if you so desire).
http://www.w3schools.com/HTML/html_colornames.asp