Out of all the myriad of problems that now desperately need solving as a result of Brexit - spending time and money considering the colour of the passport is rather like the proverbial rearranging of the deckchairs of the Titanic.
Here's what I think:
New passports (after we have left the EU) should be blue, but otherwise identical to current passports.
There you go, fixed the issue without spending any time and money. All we need is a minor change to the order to the passport book manufacturers.