No emoji. Color picker. Sounds good.
Although, I agree with Chris that yellow should be the default. In literature highlighting systems, green is used for the passage’s main idea or big takeaways. In argument mapping, green is used to highlight supporting arguments. In law school, for case briefs, yellow is used as the default. The reason why is that, in the days before computers, law school students could buy replacement yellow markers individually instead of buying a whole pack (which they would have to do if they used, say, green as their default).