No, this isn't correct. For instance, any drop has a chance of being "painted" and among painted items there's an equal chance of any of several colors. However, because of demand, Titanium White painted items are much more valuable than, say, Burnt Sienna.
That's a weird quibble. If there are ten variants and only one of them is Titanium White, the desirable one is only available in 10% of the drops and is thus rare. Yes, desirability plays a role, but it's still rare.
Desirability determines demand. Rarity determines supply (relative to the entire value on the market). Both are taken into account to determine the price.
My favorite place to view this is Path of Exile in game economy. They do have random loot boxes for cosmetics, but I'm talking about the actual in game economy around items that power up your character. None of which can be bought for real money and instead require time to farm them (RMT does exist but is against ToS and can get you banned).
There are extremely rare items that cost nothing because they aren't used. There are quite common items that have decent value because of how high the demand it. Technically the game doesn't even have a currency, but players found a small set of common but also highly useful items that have become the defacto currency so much so the developers even admit they now count as currency.
Its not really a quibble, but you aren't wrong either. Its true that valuable items will be rare, but it isn't true that all rare items will be valuable. Both the titanium white and the burnt sierra colors are equally rare, but one is much more valuable because demand is not equivalent.
Sure, but there is a ceiling on any item that isn't one of the super super rare items that barely anyone goes for except the extreme collectors. (Usually an item that goes for much cheaper but has a special tag on it that's only visible to you.)
In apex there have been posts from people who have spent thousands to try to unlock a particular item. Literally opening the same random box 400 times and failing to get the item.
This is not always the case on apex legends, where some limited offer items are only obtainable from loot boxes.