On virtually any iMovie you can add your own text on top of videos right within the application. On the current version, all you have to do is click the point on the clip you want to add text, click the "Titles" button above your library or whatever, choose the title format or animation you want, drag it to above your clip, and there it is. And with many of them, for greater control over fonts and colors, on the drop-down font list select "Show Fonts..." If you want to change the color of just certain parts of your title, highlight said part and change the color of the highlighted text.
Or unless you want even greater control than what iMovie offers...