- Zodiac's letters are written in blue ink. The first is read by the editor who is wearing a blue shirt.
- When reading the decoded cypher, thus introducing Avery and Graysmith to each other, they are both wearing blue shirts.
- Robert is wearing a blue shirt in every scene he's in.
- In Morti's Graysmith introduces to Paul Avery a blue drink called an Aqua Velva. In the following shot with the multiple empty glasses the song "Crystal Blue Persuasion" by Tommy James & The Shondells.
- Melvin Belli's suit is navy with a light blue shirt.
- The woman who pulls over to help a Zodiac victim is wearing a blue jacket.
- Dave Toschi's suit is light blue when reading Zodiac's Halloween card.
- Paul Avery's shirt on the plane is blue.
- Arthur Leigh Allen's work coveralls are dark blue.
- When Robert is on the phone while his kids are going over his files, the shirt he wears and the phone are both blue.