macOS Sierra Preview: Have the highlight colours for PDFs changed? The highlight colours in my Preview application appear darker just after updating to macOS Sierra Version 10.12.1. Is this a change introduced in the update or is this a bug?

Note: I'm using Preview Version 9.0 (909.6)
Yes, the colors have darker shades in macOS Sierra, compared to El Capitan. What you are seeing is not a bug.

This is true for all colors (yellow, blue, green, purple, and pink).