I personally don't believe in the alignment system. The debates on what Solas' alignment would be just enforces it. The alignment system is too constricting. It tries to put people in little neat morality boxes they don't fit in. Most people are too subtle, nuanced, and complex to fit into neat little "s/he's always lawful/chaotic" or "s/he's always good/evil" or "s/he's often/always neutral in some way."
Take Alistair, for example. He's one of the most tight-laced companions we've ever had (second only to Sten), very hard up on duty and honor. He's also a good person who wants to help people. So, it would make sense to classify him as Lawful Good, right? Well, no, Alistair has a chaotic streak when it comes to religion. He couldn't stand Chantry life or his Templar training because single-minded religious devotion just didn't appeal to him. (He also used to act up because of it.) So, would we classify him as Chaotic Good? Well, no, he's not THAT chaotic. So, would we put him as Neutral Good? Well, maybe, but he's still Lawful in most areas of his life, with only the little chaotic streak.
Likewise, Solas is all over the map. He may be "lawful" in that he has a personal code of conduct and sees the world in a very orderly way, but he doesn't believe in using laws or rules to constrict people's freedom. As the possible "God of REBELLION," he also clearly thinks that following rules is optional. So in that case he could be Chaotic, but that's hard to figure since he is so proprietorial. He could be classified as "Neutral" because he's a situational lawful/chaotic soul, but that still doesn't seem to cut it. As for morality, he could be described as "Neutral" because he keeps himself at a distance from others and is willing to incorporate questionable means to achieve his goals. On the other hand, he could be classified as "Good" because he uses said questionable means to help people. He also gives more approval for helping others than perhaps any other "good" companion, and his end goal is always to ensure the masses have free agency and the right to pursue their own happiness. But "Good" doesn't seem to cut it either because he'll still incorporate and/or agree with horrible methods if it achieves the desired result. I think the closest alignment to Solas might be "True Neutral" since he has bits of all traits (situational lawfulness and anarchist, a good soul who incorporates evil means while remaining outwardly neutral and apart)... but he's also not truly neutral because he's an activist that feels very strongly about many subjects.
So, I'm not sure I know what his alignment is, or if there even is one. As the trickster God of Rebellion, I imagine Solas would go out of his way not to fit neatly into a little box, and might even feel insulted that a person wanted to put him there. (Remember how offended he gets after the Orlesian Ball, if Lavellan tries to talk to him about "our people"? And he pretty much ends the conversation expressing the wish to be seen as an individual and not just "another pair of pointed ears"?)
P.S. As for what colors to fit him in, I always go for earthen green/brown in the beginning, and more vibrant red later on. (To symbolize his inner passion and fire coming out over the course of the game. "Scotch bonnet candy" indeed.)