The key word is motivation. Esentially, the function of establishing a visual vocabulary for your piece is to make sure the transitions and effects you apply aren't arbitrary, but instead inform the viewer. This information can be expository (i.e. lets the viewer know who's speaking), but can also be furthered to layer ideas through juxtaposition. The latter is more complicated and difficult to explain concretely, however, if you make sure that every choice you make carries with it some meaning, and then stay consitent with these choices, you'll soon find ways to present your content in more meaningful ways.
Everything you make will have an opinion, just by the virtue of you making it. You might say, 'well, mine is just an informative video, and doesn't really have an opinion'. Unfortantely, art can't escape bias because when you show something, you're always not showing something else. Knowing this, we understand that it's better to go into a project with an argument in mind, rather than allow a misconstrued argument to form by itself.
Every opinionated piece will always have three basic components: A thesis, an antithesis, and a synthesis. The idea is that the thesis and antithesis are both problematic as the argument for the antithesis is always contained within the thesis and vice versa. The synthesis can be considered the combination of the two, a solution to the dilemma.
Once you've outlined your argument, you can begin to develop a simple visual vocabulary for each component. Assigning a colors should be your first step (i.e. use blue when showing the thesis content, red for the antithesis content, and purple for the sythesis, the result of the two).
Visual vocabulary isn't limited to colors and animations.
By being consistent with your use of colors, animations, and fonts, you devlop a visual shorthand for the audience that can be used to layer ideas.