So, I've been trying to properly use は and が. It is often said, online, to differentiate using topic and subject. That has always confused me, so I noticed a way to use it that seems to always follow the following rule: use が if someone is unaware of the thing you're talking about, and は if everyone is aware.
For example, you would only say "there's a car" if you know someone was unaware of the fact, therefore, "車があります" is used.
Alternatively, "the car is green" is used when everyone is aware of what car you're talking about, therefore, "車は緑です" is used.
Is this a functional guideline, or are there glaring exceptions that I have not thought of?