Earlier I wrote that in chapter 42, Ishmael challenged the association of the colour white with positive qualities such as purity, sublimity, honour... by suggesting that white could cause/ intensify fear. Now, reading it again, I still don't know what the chapter means, or what Melville means to say, with all the ideas and symbols, but I'd put it differently: if we put aside Daggoo's remark 2 chapters earlier about black, and don't assume that chapter 42 is necessarily about race, what Ishmael does is writing about the 2 different, or opposite, effects of the colour white, thus pointing to the elusiveness, indescribability, ungraspability and, most importantly, the duality of the colour. It can be beautiful, it can also look bleak and desolate. It can be sweet, it can also heighten dread and terror. It can be a colour of innocence and purity and sacredness, it can also be a blankness, an absence of colour. White has a duality the way the whale has double meaning: both God and Satan, both good and evil, both a victim and a sea monster, both a dumb brute and a force of nature...
The point is not which of the 2 sides is the meaning. It's the duality that is the point.
Moby Dick is built upon a double quest: Ahab's is to hunt a whale, Ishmael's is to understand both the whale and the hunt. Ahab simplifies (seeing Moby Dick as the personification of all the evil in the world, giving up on life and concentrating all of his time and energy on the pursuit of the whale), Ishmael elaborates and complicates (combining book-learning and actual experience, bringing in history, religion, philosophy, biology... to get to know the whale). Ahab has 1 obsession and cares about nothing else ("This lovely light, it lights not me; all loveliness is anguish to me, since I can ne'er enjoy"), Ishmael embraces everything and finds great joy. And, as John Bryant and Haskell Springer put it, the novel's structured around 2 consciousnesses, with Ahab as a force of linearity and Ishmael a force of digression.
I think of Moby Dick in terms of number 2 (pairs, doubles, opposites, foils).