As someone with a solid education in art/design, the most important thing I can think of is readability.
That is, making the words and main image you use readable from a distance by making them big enough.
Other than that, you should really just stick to traditional painter's principles of visual design since that is what I find the most effective even just with 3-color posters.
If you want to learn more, look up the work of James Gurney and especially his book 'Color and Light - A Guide for the Realist Painter'.
To start you off, look up his own 'Windmill Principle' (https://gurneyjourney.blogspot.com/2008/04/windmill-principle.html
), which helps a ton with understanding how to make text and images contrast against one another effectively.