Judy's right. Putting the different colors on different slides won't really work. Your "workaround" shows four different versions of the next slide. So that means four slides, and that means you would have to make four distinct courses. That's because they are colors, not background colors. Background colors (in the sense David is using background in the video), really means Master, or template colors. You create them once, and they are the same for each new slide you create.
You can also save some time by consolidating some of the triggers. In the sample, there triggers that fire when the slider moves, when the slider changes, when the background variable changes - all very confusing. Remember, for troubleshooting now, and maintenance later, consistency is your best friend. I rearranged the triggers to fire on consistent and logical actions, then grouped and organized them for ease in finding and managing them. I disabled some that seem not to be used, and added the necessary ones to allow choices to be made by clicking directly.
See the New scene in the attached sample, and ask if you have any questions.
