I saw this on another Magix site about adding a color panel and wanted to help with an answer. The contributor stated that "he opened a new file in Windows Paint and edited the attributes to a full screen size for HD, since that is what he worked with the most and then created about 10 different colored background, saving each to a folder for backgrounds."
I tried it and it worked easily. I saved the backgrounds to "My Pictures" where I could quickly get them and add to slideshows.
Photostory has a set of colour images (patches) already, these can be used as shown below, or you can insert your own colour images where I have done with the built in colour patches.