Wow! I'd think this would annoy most users. You could do it by creating a new set of styles and applying them as appropriate. Perhaps you could give us some more information as to exactly why you'd want to do this. What is the audience and their desired workflow?
Whenever a new version of content is published, the changed content should be shown in a different color. That means, the content shown with a different color this time may be shown in the default color in the next version.
You can use track changes. Set track changes and publish with the tracked changes visible. Then accept all changes and start again.
It won't be pretty, but it may do what you want.
With track changes enabled and published, it shows both the old content and new content, but doesn't differentiate with colors.
old content: "This procedure defines specific requirements for"
published content with change: "This procedure defines defined specifics requirements for"
I might think this would cause you a maintenance nightmare. At minimum, you are adding more work for yourself.
One way I accomplished allowing my users an avenue to see what was changed was to insert a DHTML drop-down at the top of the topic. I linked the DHTML trigger to a small image. Then if they wanted to see the change log for that topic, they clicked the small icon and the changes were revealed.