Alongside my mission score, sometimes there are red X's, sometimes there are studs that look like pearls and sometimes there are gold stars.

Each OFP mission has it's own definition of what constitutes a distinguished, average or poor score for that particular mission. Poor scoring will result in 1 to 3 red X's being displayed alongside your score. Average scoring will result in 1 to 5 pearl-like studs being displayed alongside your score. Do very well and you should see 1 to 5 gold stars appearing alongside your score.

Scores can also be negative. This can occur for a variety of reasons, such as killing friendlies or not achieving objectives.

By the way, earning a high score while playing in cadet mode will result in earning less studs and gold stars than you would otherwise have earned achieving the same score in veteran mode.