Even in strikethrough the line color is same as the text.
The reason you can't change the color of the strikethrough line is that there simply isn't enough demand for it. All of the examples I've seen in HTML the strikethrough is same color as text.
To implement it word would have to store a separate color attribute for the strikethrough. Unicolor strikethrough has been carried over from typewriters and dot matrix printers, there just hasn't been enough demand for the feature.
Heck, even Office 2010, the HIGHLIGHTER feature is STILL limited to the original 16 EGA (or was it CGA, 1 can never remember) resolution colors while all other color attributes have (finally) been updated to same ~2 million color level. In one release of
Word 3 different color attributes supported 3 different color resolutions.
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http://office.microsoft.com/en-us/outlook-help/learn-where-menu-and-toolbar-commands-are-in-office-2010-HA101794130.aspx - Silverlight applets Mapping 2003 to 2010 commands for all Office apps.
http://office.microsoft.com/en-us/training/HA102295841033.aspx - flash applets mapping Office 2003 menus to 2007 ribbon.