If you use a browser inspector tool like Firebug, or the built-in inspector in other browsers, you can see that the class for widgets is widget-title. This means you can change it by adding this code to your custom CSS:
Here are some great reference articles where you can learn more about how to find your site’s CSS classes and IDs, to be able to make changes:
P.S. I did take your advice earlier and found the css selector widget-title, and tried to use it to change the colors. The difference is that I didn’t know I was supposed to precede the selector with a single dot as you did.