The type of element selected will vary what color variables you can amend, color is changed by selecting an element and clicking Style on the tools palette.
Text
When text is selected, within the Style menu it is possible to change the text color, the background color, border color and an overlay color. Text color and background color are self explanatory, however border color and overlay not so much. Border color can be defined, but if there is no border on the selected element you won't see a change. Just under color settings are border settings where you can change the width of the border, by default this will be zero on most elements. Until you define a border width you won't see it to define a color. The Overlay color is the exact opposite of the background color, it appears over the top of everything.
Image
When an image is selected, within the Style menu it is possible to define background, border and overlay colors. Remember with background colors that the background of the image has to be visible to change it. For instance, if you are using JPEG images it isn't possible to have a transparent background in them at all, so trying to change the background in SiteBuilder will not make a difference from whatever the image is currently. If this is all new to you and not making sense, this article covers the basics of image file formats.