The multipolygon is a school, and the inner ways are buildings, sports pitches, car parks etc. I’ve made sure I tagged the relation rather than the outer way, but as far as I can see the inner ways need tags.
What should I do to keep JOSM happy, whilst still being able to identify the various elements in these multipolygons.
BTW, the area is rendered perfectly in OSM. I’d just like to clear the validation error.
There’s no reason use a multipolygon in that situation: inner members are “holes” in an area, and your school area probably doesn’t have holes.
What your multipolygon relation currently says is: There happen to be a pitch, a parking area, and a random building surrounded by the school, neither of which are part of the school. Probably that’s not actually the case.
If you just add the amenity=school and name tags to the outer polygon and delete the relation, the tagging will be correct. Just in case you do want to add them to a relation anyway (even though it’s not really necessary for such a simple case), what you are looking for is a site relation.
Oh, and the validator doesn’t actually know all this - the warning is just a bug caused by the fact that parking and school use the same color in JOSM.