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This map answers one question — my people came from here, who holds the paper — and there are three different things you might have in your hand when you ask it.
A blank on this map is a gap in the map, not a gap in the record. The shelf is deep for Croatia and thin everywhere else, because somebody walked a Croatian catalogue parish by parish over months and nobody has done that anywhere else. Most of the world's dots here are administrative districts — the county, the département, the Kreis — put on the map because that is the unit records are filed in, and left hollow because nobody has been through them.
So a grey dot means «nobody has looked here», never «there is nothing here», and the two are kept apart everywhere — including on a page that measures exactly how incomplete this is, country by country.
Every dot is coloured by what it would cost you to look right now — free images, images behind a free login, an index with no images, a paywall, a catalogue card for paper nobody photographed, or paper in a building. That is the question a researcher actually has, and it is the only thing the colour is ever used for.