You may be able to change the background color of an image, such as a logo, using an image editing program or application, like Gimp, which is free, open-source, and can be downloaded from the link below.
http://www.gimp.org/macintosh/
Once you've downloaded and installed Gimp, you might be able to achieve the desired effect by opening the image you'd like to edit and then either using a selection tool to "cut out" the background entirely and then replace it, or by using the "Fill" tool, which will fill a particular section (as long as it is the same color throughout) with the color you desire.
Additionally, you may be able to find some great tutorial videos or guides online, on a variety of Gimp related topics, including this one, by doing a search with your favorite search engine!
Good luck and I hope I helped you!