You have two problems.
The first is that you're making your changes in StarterGui, but StarterGui is not what a player sees. It is simply a holder for GUI objects that will later be cloned into your PlayerGui. The PlayerGui is what you see.
The second is that your title
variable equals an object, not a property. In this case, you need to use the BackgroundColor3
property.
Just put a local script inside the GUI object with similar code;
3 | script.Parent.BackgroundColor 3 = Color 3. new(math.random(),math.random(),math.random()) |