Can I assume that every creation in this world was first manifested in the world during the first six days of the Genesis, which therefore includes color? So was color (the spectrum of hues and shades, etc.) a phenomenon that was created during Ma'aseh Bereishit, or not until the rainbow in Noach's generation (by the flood)?
Pesachim 54A says the rainbow was created on the sixth day:
Ten things were created on the eve of the Sabbath at twilight. These are they: the well, the manna, the rainbow, the writing and the writing instrument[s], the Tables, the sepulchre of Moses, the cave in which Moses and Elijah stood, the opening of the ass's mouth, and the opening of the earth's mouth to swallow up the wicked.
In order to have the rainbow you need color, so the answer appears to be yes.
Whatever the argument of the creation story (that God controls that world, that there is order to the world, etc.), that argument is being made to a generation of slaves without technology. Assuming the "creation of color" is truly distinguishable from the creation of light, I don't think this it is a concept that would have been understood by the generation leaving Egypt, to whom, at least on a pshat level, the Torah was pitched.
By way of analogy (It’s not perfect, but I think it works):
I’m currently reading a book by R Natan Slifkin about reconciling Science and the Torah. In his discussion of evolution, he quotes R’ J. Hertz:
... condensed from the pages of one of the leading biologists of the age (Haeckel):-
“Monerao begat Amoeba, Amoeba begat Synomoebae, Synomoebae begat Ciliated Larva, Ciliated Larva begat Primeval Stomach Animals, Primeval Stomach Animals begat Gliding Worms, Gliding Worms begat Skull-less Animals, Skull-less Animals begat Single-nostrilled Animals, Single-nostrilled Animals begat Primeval Fish, Primeval Fish begat Mud Fish, Mud Fish begat Gilled Amphibians, Gilled Amphibians begat Tailed Amphibians, Tailed Amphibians begat Primary Mammals, Primary Mammals begat Pouched Animals, Pouched Animals begat Semi-Apes, Semi-Apes begat Tailed Apes, Tailed Apes begat Man-like Apes, Man-like Apes begat Ape-Like Men, Ape-Like Men begat Men.”
Let anyone who is disturbed by the fact that scripture does not include the latest scientific doctrine, try to imagine such information in a biblical chapter.
From various meforshim (from memory as I do not have the sources available) such as Nechama Leibowitz, Rabbi J. H. Hertz, Rabbi Wein, Rabbi Hirsch.
Given that the first mention of watering the garden was from the mist, it could have been that the heavy moisture in the atmosphere before the flood did not allow for a rainbow. During the flood, all the water in the atmosphere condensed leaving the atmosphere the way it is now (which allowed a rainbow to occur). Since Noach had been shut up in the ark the whole time, this was the first time that he could have seen one. Hashem then told him the meaning of the rainbow, especially tied to the fact that all the water in the atmosphere had flooded the Earth.
Rainbows could have existed before, but this time it was being given a religious and symbolic meaning. Once it was given this meaning, seeing it would bring to mind what had happened and what Hashem wanted people to remember. @MichaBerger points out that this is the view of the Ramban.
As Rav Hirsch says at Noach 9:13
It is by no means necessary to assume that there had been no rainbow and to place it in connection with the atmospheric changes which occurred after the Flood. Just as Hashem showed Avraham the starry heavens ... My bow, I have long ago placed it in the clouds, henceforth it shall serve as a sign of the covenant between Me and you.