After much exasperation using the program myself, I decided to have my friends try to make a basic slide in OpenOffice Impress, program roughly equivalent in fu

Instructions:

1. Change the slide background to green
2. Add some text with a white background to the slide
3. Take screenshots every time you do something 

This ended up taking 5-10 minutes for everyone who tried it.  My test subjects were CS, EE, or CSE majors with extensive computer experience (but had never used OpenOffice Impress before).

Universally (probably due to previous experience with PowerPoint), all test subjects right clicked the background and went directly to "Slide Design".  However, this is not what you want!  I did the same thing as well...

After this initial mistake, the next mistake was that people then went to "Slide Layout" or "Set Background Picture" expecting to be able to set the background color.  Their hopes, however, were dashed by a lack of that option.  Everyone skipped the correct "Page Setup" dialog box option.

One test subject decided to modify the slide master, and was messing with the "Master Pages" and "Layouts" views on the task pane (right side of screen) to no avail.

Another test subject went to Format->Area in the menu, pulling up this dialog box:

This lets you set a color...but it's the fill color for drawing shapes, not the fill color for the entire background of the slide!  All test subjects who got to this point were complaining about the task.

The correct way to do it, which all test subjects eventually stumbled on (within 5-10 minutes), is the format->style and formatting menu option.  This has the background color available as a property:

Selecting the background color involves right clicking the "background" button/text in the list (which is not hinted to at all)

Suggestions for improvement - when you right click a slide - have the either a button on the menu that goes directly to the background dialog, or simply have the Styles and Formatting dialog be an option.

Also, universally, no one (including myself) went to page setup to set the background color.  Since this is used in most other programs for printer settings, I don't understand why the background color would be tucked in there, but it is.  This is available when you right click a slide.  However, it makes more sense to be inside "Slide Design."

Conclusion:

Open Source Software like OpenOffice Impress is feature-rich and competitive with commercial applications like Microsoft Office.  They fail when it comes to UI testing, as they don't have the capital to organize large-scale user testing, or a non-developer user base willing to provide detailed feedback about the software.