My site has two main content types, each with their own taxonomy tree. I would like to display teasers from both types in the main content area of the front page, interspersed according to the posting date. However, I would like to give readers a clear indication of which type each teaser comes from. I was thinking of adding a coloured bar running down the left side of each teaser for all its height, with the colour depending on the type and maybe the type name appearing vertically inside the colored bar.

As an alternative, I could use different background colours for teasers of each type.

Can somebody help me in doing any of the above? I'm not a CSS type and I don't know how teaser theming works.

The good news is you should be able to any of these things.

I'm assuming that by two content types you mean two different node types? Node formatting is controlled in the template file for the node. The template file lives in your theme directory and takes the form: node-{type}.tpl.php. Theming handbook at Drupal.org has lots of great theming info, though you may need Google to find some of what you're looking for.

This describes how to determine if the node template is being asked to display a page or a teaser: http://drupal.org/node/53464

There's the $is_front theme variable if you want something specifically for the front page.

Also check out this: http://www.drupalbook.com/. The sample chapter covers Drupal theming.