Those two sentences are based on the differences that scholars have observed in the ritual practices associated to Olympian and Chthonic gods, i.e. respectively the sky and earth deities. Indeed, there seemed to be a tendency to sacrifice white animals on raised altars to the heavenly gods, and black animals in pits to the subterranean ones. See for example what W. Burkert says about these practices in his book Greek Religion:
White cattle or sheep may be sacrificed to the heavenly ones and black
to the subterranean ones.
Frequent is the antithesis of offering pit or ground level hearth and
raised stone altar, corresponding to a Chthonic and an Olympian
For the first sentence, he provides a (non-exhaustive) list of references, including passages from the Iliad and the Odyssey:
Bring ye two lambs, a white ram and a black ewe, for
Earth and Sun, and for Zeus we will bring another
Hom. Il. 3.103
And do thou earnestly entreat the powerless heads of the dead, vowing
that when thou comest to Ithaca thou wilt sacrifice in thy halls a
barren heifer, the best thou hast, and wilt fill the altar with rich
gifts; and that to Teiresias alone thou wilt sacrifice separately a
ram, wholly black, the goodliest of thy flock. But when with
prayers thou hast made supplication to the glorious tribes of the
dead, then sacrifice a ram and a black ewe, turning their heads toward
Erebus but thyself turning backward, and setting thy face towards the
streams of the river.
Hom. Od. 10.524–7
As well as a passage from Eusebius' Praeparatio evangelica:
For gods of earth and gods of heaven each three:
For heavenly gods pure white; for gods of earth
Cattle of kindred hue divide in three
And on the altar lay thy sacrifice.
Euseb. Praep. ev. 4.9.2
Explicit mentions of sacrifices of white animals to Zeus are provided by J. W. Hewitt in his paper The Propitiation of Zeus, in which he gives some examples. One is a marble board found in the Magnesia region of Greece, of which you can see the ancient greek text in Leges Graecorum Sacrae e Titulis Collectae (p245, N.82) by Prott and Ziehen, who comment (text translated from Latin):
The white color of the sacrifices is best suited to the worship of the god of nature on the top of the mountain.
Another example given by Hewitt is from Demosthenes, and relates to a sacrifice to Zeus Ktesios:
To Dionysus pay public sacrifices and mix a bowl of wine and set up
dances; to Apollo the Averter sacrifice an ox and wear garlands, both
free men and slaves, and observe one day of rest; to Zeus, the giver
of wealth, a white bull.
Dem. 21, 53
However, it should be noted that, as the Wikipedia page you linked says, a subterranean form of Zeus (called Zeus Chthonios, among other names) was worshipped at various times and locations in Ancient Greece, to which black animals were typically sacrificed. For more on this, in addition to the above-mentioned book by Burkert and paper by Hewitt, see for example Olympian and Chthonian by S. Scullion.