A lie of omission, which is what you are describing, is still a lie and thus has no truth in it.
"The best lies always contain a grain of truth."
Or rather, let me use an example from the tv show "Sherlock," with Benedict Cumberbatch.
In season 2, Moriarty is convincing most people that Sherlock is a fraud, and he dupes a reporter into thinking that (a reporter that Sherlock utterly offended so she had motivation.)
The lie, Sherlock Holmes is a fraud. The premise and the clause is an outright lie. But it was one Watson said people would believe because everything else in the story she reported was true.
For Solas, the advice he gives the Inquisitor and the other companions make sense and take into account human/elven nature. Which is why I find his dialogue with Cassandra so enjoyable, because he expresses surprise so often that Cassandra avoids the corrupting influences of power that he expects.
The truth, he is fighting alongside the inquisition, offering advice and his expertise on the Fade. He urges the Inquisitor in Haven to find the artifact that created the breach and offers advice to other members of the Inquisition. He even offers to teach Sera about ancient elves and maybe how to unlock power that comes with being an elf, which she rejects out of hand.
The lie in his case is interesting, especially since he never outright tells us a falsehood, but gives us lies of omission. He urges us to find the orb but leaves out that doing so and finding it intact would enable him to destroy the world as he saw all its denizens as little more than tranquil in terms of being actual people. Nor does he say he'll fight bring order back into the world, rather he tells the inquisitor that he'll stay, at least until the mess is dealt with and says that he's an apostate surrounded by Chantry forces, which he is.
His lies are lies of omissions, since I don't recall him ever throughout the game saying an outright falsehood. But he is still deceiving the inquisition about his motives.
He's a very complex character.