Granite Porosity Varies by Colour
One Word, Many Rocks
Commercial granite covers true granites and a range of other igneous and metamorphic stones sold under the same heading. Their mineral composition, grain size and pore structure differ considerably, and so does how much liquid they take up. A statement about granite in general is a statement about an average that describes nothing in particular.
The Broad Pattern
Dark, fine grained stones tend to be dense and absorb very little. Lighter stones with visible crystal structure and coarse grain tend to be more open. There are exceptions in both directions, which is why the pattern is a starting point for a question rather than an answer.
Test the Actual Slab
Ask the yard or fabricator for an offcut, or test a discreet area of the installed top. Water left for fifteen minutes either darkens the stone or does not. That single observation is worth more than any category level advice, and it can be repeated annually in a minute to decide whether resealing is due.
What Sealing Does and Does Not Do
An impregnating sealer occupies the pore structure so liquids sit on the surface long enough to be wiped. It does not create a film, it does not make stone stainproof, and it does nothing at all against etching, which is a chemical attack on the mineral rather than absorption. Dense granites are largely unbothered by acids anyway.
Where Marks Actually Happen
Oil is the practical enemy for an open stone, because it is slow, it is often not noticed, and it migrates. The area beside a hob and the strip in front of a sink account for most real world staining. Those are the two places to test and the two places worth treating even if the rest of the run is fine.
Buying With This in Mind
If a low maintenance surface matters more than a particular look, the question to ask at the yard is which of the slabs in your price range are the densest. Yards know, and it is a question they are rarely asked. It narrows a bewildering choice down to a manageable one in a single conversation.