The theme you're using displays excerpts on the main page. Automatically produced excerpts are plain text only (no code), so changing the color of the post itself has no effect on the main page. To change the color of the excerpts you need to create your own custom ones, with a little bit of coding, in the Excerpt module of the post editor:
<p style="color:#222;">YOUR CUSTOM EXCERPT HERE</p>
(222 is an example - you can change it.)
If you're not seeing the Excerpt module while on the post editor screen, click Screen Options (top right) to activate it.
But this isn't enough, because there's no equivalent way of changing the color of the list items in your widgets (Recent Posts, Categories etc). To change those, as well as to change the color of the Excerpts once for all, you need the Custom Design upgrade.
So the solution is to use a very light or a very dark bg color.