Blue passports
Croatia has a dark blue passport.
Result: Neither
With a separate British passport, travel to the EU will become considerably harder.
Remember that freedom of movement works both ways, so holidays will require more paperwork (Travel Visa), and more time waiting at border control while that paperwork is checked.
The EU unified the passport design in 1981, but it was only a recommendation.
For Croatia, the burgundy red was associated with the old Yugoslav passports (and the old communist regimes), so their government kept the dark blue passport colour when they started issuing EU passports in 2015:
If you disagree, or would like to add more details, please let me know.
You can suggest a reason for leaving the EU, or look at the other reasons.