I have made a simple blog in Wordpress, using a simple theme. I have only ever used Wordpress once before, and that was installing a blog with a theme, and that was it. I also am very rusty with PHP

Both I and my girlfriend will edit the site. I would like to highlight the blocks of text we each write, perhaps with an indent & different color text.

Obviously, I can do this by adding a css class for the author, and using "get_the_author();" to set the class for the complete article - although I have not yet worked out where to put this, I am sure I can find the appropriate content div.

What I would like to know - not having had much WP experience - is, is there a way that I can flag later edits, or additional text within a single article, automatically, depending on who made the additions?

ie, I log on, write 3 paragraphs and save the article. It would be shown in red. Then my girlfriend logs in, with a different WP user, and adds 2 paragraphs to the same article. These would be shown in blue.

I realise that this may be beyond the reach of Wordpress, but if there is a direction to investigate, what is it?

Well here's something like what you want -- it shows a revisions list, with the option to view diffs, although there are some options that might bring the behaviour in line with what you want: http://www.movingtofreedom.org/2010/07/30/wordpress-plugin-post-revision-and-diff-viewer/

Basically I searched for WordPress revisions and WordPress revisions plugin and that was one of the first results -- I'm sure if you dig around there'll be something that will do exactly what you need.

Cool, thanks - will have a look at it.

Decided, in the end, that it is easier to add a custom tool to the rich text editor, which will add a css class to the current element.