Ryanair's order for Boeing 737 MAX 10 planes - how will this affect the Buzz and Malta Air fleet?

unless I am mistaken, the vast bulk of the fleet registered under Malta Air has a Ryanair livery.

Vast bulk doesn’t mean all and Ryanair is the core brand. I would expect the vast bulk of the new ones also to be liveried in Ryanair.

A relatively small number of B38Ms were delivered in Buzz (eight aircraft) or Malta Air (six aircraft) livery, with all subsequent aircraft for these carriers wearing standard Ryanair colours. Lauda Air Europe A320s all wear that operator's livery and there does not seem to be any plan to change this.

As for 737-10s, we don't know for sure but the evidence would suggest that most, if not all, will arrive in Ryanair livery. As has been stated, that is the core brand and Ryanair won't want to dilute it, or indeed complicate any moving around of aircraft between the different arms of the company, as has happened a good bit with the 737-800s.

Polish and Maltese-registered aircraft populate lots of FR routes that do not touch those countries. I have noted that Irish-licensed pilots fly those aircraft too, so they are in effect integrated into the general Ryanair network. At Dublin on any given day there are streams of 9H- and plenty of SP-registered 737s operating Ryanair flights from and to a plethora of places. Malta Air has 179 aircraft at present so it is rather more than a replacement for Air Malta (which lives on as KM Malta Airlines anyway). You can look at the flight history of any individual aircraft on FR24 if you wish.