For just a little more info on the Ithryn Luin, they both wore 'sea-blue', and had went into the East (i.e. Rhûn) with Saruman, and never returned. It is possible that they pursued their purposes there, or perished, or were ensnared by Sauron and became his servants. Or, they could have been corrupted like Saruman, searching for power. Alatar was chosen to go to Middle-earth by the Vala Oromë, and he took Pallando as a friend. Other notes indicate that Pallando was also a follower of Oromë (this replaces his previous ties to Námo and Nienna).
Tolkien said "they went as emissaries to distant regions, East and South, far out of Númenórean range: missionaries to enemy-occupied lands, as it were. What success they had I do not know; but I fear they failed, as Saruman did, though doubtless in different ways; and I suspect that they were founders or beginners of secret cults and 'magic' traditions that outlasted the fall of Sauron."
On an etymological tangent, it is possible that the names Alatar and Pallando meant Great Lord and Wide Gate, though those are entirely my speculations and have no attestations.