According to the Cambridge Dictionary, "A pastoral piece of art, writing, or music represents the pleasant and traditional features of the countryside." When Keats refers to the urn as a "Cold Pastoral," he is describing the function of the urn as an object used to communicate a story about the Grecian countryside. For example, earlier in the ode, Keats meditates on a part of the urn that illustrates a woman leading a calf to slaughter, which was a common event in the farmlands of Ancient Greece and often features in Greek literature:

Who are these coming to the sacrifice?
To what green altar, O mysterious priest,
Lead'st thou that heifer lowing at the skies,
And all her silken flanks with garlands drest?

Calling the pastoral "cold" is likely a reference to the temperature of the urn itself; if you've ever touched clay ceramics, you've likely experienced the cold sensation of a non-living object. In addition to this very literal meaning, Keats is also referring to the urn's preservation of the events it illustrates. For example, earlier in the poem, Keats meditates on an image of two lovers leaning in for a kiss below a tree:

Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,

Keats muses on the power of the urn to defy contemplation. He compares the urn to eternity. In trying to understand eternity, we can gain no answers to our many questions by contemplation and musings. Similarly, we can gain no answers to our many questions about the images painted or carved or raised in relief on the urn by contemplation and musings. This is because, like with eternity, facts are not available for the scenes on the Greek urn. Even its purpose is obscured over time. Was it a funeral urn for cremated ashes or a wedding urn for celebratory wine, or something else?

Keats addresses the urn in an apostrophe that is a play on words, "Cold Pastoral!" meaning the pastoral (country) wedding scene is "cold" because it unfeelingly won't communicate its truths and it is "cold" to the touch because it is "marble men and maidens." "Cold" also means that the everliving (ancient) yet lifeless urn has no warmth from flowing blood and changing events and fortunes.

The cold pastoral wedding scene, under "forest branches" and "leaf-fring'd" garlands, tells the tale that "Beauty" is all that endures through the countless ages; all else crumbles to dust. It tells the "Truth" of the lives captured for a moment thereon and the "Truth" of life's fragility, its fragile, changeable condition. This "Truth" is seen in the "Beauty" that remains because the urn and the lives held on it still remain. The poet understands that this "Beauty" and "Truth" are all that is knowable, thus all we need to know.

Why does the speaker call the urn "Cold Pastoral" in "Ode on a Grecian Urn"?

In answering this question, we first of all need to understand the meaning of the word “pastoral.” A pastoral is an idealized portrait of rural life, often populated by mythological figures such as nymphs and dryads. One such pastoral scene is depicted on the side of the urn that forms the subject of Keats's poem.

All the familiar elements of pastoral are openly on display. We have “deities and mortals,” maidens, “pipes and timbrels,” and a “Fair youth” sitting beneath the trees. This is a lively scene with much joy, laughter, and sweet music.

Yet at the same time, this pastoral scene is cold in the sense that it does not, and indeed cannot, come to life. The “soft pipes” may play on, but of course, we cannot hear them. And the “happy boughs” will never shed their leaves, nor will they “ever bid the Spring adieu.” In the scene depicted on the urn, it will always be springtime.

What Keats seems to be driving at here is the timeless nature of great art. All works of art, including the urn, are the products of a specific time and place. And yet the very greatest works of art have a certain timelessness about them, which enables them to transcend the narrow confines of the historical and cultural eras in which they were created.

It is this timelessness that allows great works of art to speak to successive generations, even though they no longer adhere to the cultural norms and values of a bygone age such as ancient Greece.

The pastoral scene depicted on the side of the urn may well be cold in that it does not come to life. But the flip side of this is that it can also never die. And that is because the truth it contains—beauty with a capital B—is indestructible.