When the built-in color panel's eyedropper magnifying glass won't match your image's background color, what do you do? DigitalColor Meter will get you the exact color you need.
When adding images with flat color backgrounds (such as logos) to your desktop as wallpaper, you usually want to center the image and have the desktop background color around the image to match. Black and white usually work fine, but other colors are not sampled correctly when using the built-in color picker's color sampling magnifying glass.
Instead, once making your image the current wallpaper, open DigitalColor Meter (In /Applications/Utilities/), switch the main dropdown to show you the color's value to 'RGB as actual value, 8-bit' and hover over the color in your background you wish to spread over your whole screen and press Command+L to lock the sample for the next step.
Go back to System Preferences, open the built-in color panel, go to the Color Sliders panel (the second tab) and choose RGB sliders, then enter the three values from DigitalColor Meter into their corresponding text fields. Your desktop's background color will match the matte of your wallpaper image.
The RGB sliders are in Systems Preferences » Desktop and Screensaver, from
the dropdown choose Fit To Screen or Center, next to that appears the
background color swatch, which opens the Color Picker panel. Then pick the
second tab in the colour picker, and on that tab change the slider dropdown
to RGB sliders.
[crarko adds: I tested this, and it works as described. This previous hint takes the approach of adding a new solid color background image, but doesn't embed something like a logo in it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110130030105874