I was reading some older (1930s-era) science fiction and noticed that Earth was described as the "Green Planet". In contrast, modern science fiction uniformly describes it as the "Blue Planet". When and why did this change in terms take place?
1908 - Earth, a Blue, Shining Speck
I drew nigher to our system, and now I could see the shine of Jupiter. Later, I distinguished the cold, blue gleam of the earthlight . . . I had a moment of bewilderment. All about the sun there seemed to be bright, objects, moving in rapid orbits. Inward, nigh to the savage glory of the sun, there circled two darting points of light, and, further off, there flew a blue, shining speck, that I knew to be the earth. It circled the sun in a space that seemed to be no more than an earth-minute.
1925 - Earth, the Green Star
The novelette "When the Green Star Waned" by Nictzin Dyalhis (real name!) was first published in Weird Tales, April 1925. This story is notable for the first known mention of a fictional weapon called a "blaster", or "blastor" as Dyalhis spelled it. The titular "Green Star" is the planet Earth (or "Aerth") as seen from Venus (or "Venhez"):
That something was radically wrong with our neighbor, everybody already knew, for many years before the green light of Aerth had become perceptibly dimmer. Little attention, however, had been paid at first, for, by interplanetary law, each planet's dwellers remained at home, unless their presence was requested elsewhere. And no call had come to us nor to any other world from Aerth; so we had put it down to some purely natural cause with which, doubtless, the Aerthons were perfectly capable of coping without outside help or interference.
But year by year the green light waned in the night skies until finally it vanished utterly.
1939 - Earth, the Green Speck (suggested in a comment by @Ubik)
The short story "Marooned off Vesta" by Isaac Asimov was first published in Amazing Stories, March 1939. Unfortunately, I don't have a copy of that issue. The following quotation is from a reprinting of that story in The Best of Amazing (Joseph Ross, ed.), Doubleday, 1967:
He gazed about him. For the first time since the crash he saw the stars, instead of the vision of bloated Vesta which their porthole afforded. Eagerly, he searched the skies for the little green speck that was Earth. It had often amused him that Earth should always be the first object sought for by space-travelers when star-gazing. However, his search was in vain. From where he lay Earth was invisible. It, as well as the Sun, must be hidden behind Vesta.
When the story was reprinted in The Best of Isaac Asimov, among other revisions, the "green speck" became a "blue-white speck". The following is quoted from a 1976 paperback edition, but the text is presumably the same as in the original 1973 hardcover:
He gazed about him. For the first time since the crash he saw the stars instead of the vision of Vesta which their porthole afforded. Eagerly he searched the skies for the little blue-white speck that was Earth. It had often amused him that Earth should always be the first object sought by space travelers when stargazing, but the humor of the situation did not strike him now. However, his search was in vain. From where he lay, Earth was invisible. It, as well as the Sun, must be hidden behind Vesta.
1942 - Earth, the Blue World
The short story "Peril of the Blue World" by Robert Abernathy was first published in Planet Stories, Winter 1942 (available at the Internet Archive), well before the first satellite images of the earth. The "Blue World" of the title is indeed Planet Earth. Here is the beginning of the story:
There are those who have criticized the wisdom of the members of the First Earth Expedition in returning to Mars so precipitately, without completing the observations and explorations which it had been intended they should make. For some time now, we who were with the Expedition and knew the real reason for that return have chosen to ignore these few but noisy individuals; but latterly some of the hot-headed younger generation, but lately out of the egg and unwilling to trust to the wisdom of their elders, have begun to talk of launching a second expedition to the Blue Planet.
Therefore, I, Shapplo with the Long Proboscis, interpreter to the First Expedition, have been commissioned by the crew of the Earth Rocket to tell the full and unexpurgated story of our adventures on Earth, and the reasons for our contention that the planet must forever remain closed to Martian colonization.
If you go far enough back into SF, you'll find other colors also speculated upon for earth as seen from space. Maybe the sky was blue because other frequencies were being reflected rather than scattered, so it would look red from space; maybe it would be yellow from reflected sunlight...
Half the fun of science is when the universe surprises us. Most breakthroughs are not initially greeted with "Eureka!", but with "Well, that's not what I expected..."
[Citation of citation: Stephen Baxter, in his essay The Moon Is Hell, mentions yellow earthlight in Ray Cummings' Brigands of the Moon (1931), and "blue (or yellow or red or green) Earth" as seen from the moon in a general description of other pre-Apollo SF. I thought he had a more specific mention of red but I'm not finding it right now. Said essay, and a good sampling of Baxter's other short writings, can be found among other places in The Hunters Of Pangaea, published by NESFA Press -- unsolicited plug, I'm not a NESFA member. Yet.]