And whenever two nouns are in construct state – the first noun will be the primary.

גחלי אש (coals of fire) – the noun גחלים (coals) is the primary, and the noun אש (fire) modifies the noun גחלים (coals); that it will be burning, and that the fire requires the coals in order to exist.

And there is a difference between שני תולעת and תולעת שני, as when it says תולעת שני, the noun שני (scarlet) clarifies the noun תולעת (worm) and excludes worms of the color black. And when it says שני תולעת, the noun תולעת (worm) modifies the noun שני (scarlet) and excludes scarlet that is made from another thing. And there is a difference between them: תולעת שני – the noun תולעת is the main noun and one is able to dye with all blood of the worm, and שני תולעת - the noun שני is the main noun and one is only able to dye with the scarlet blood that is in a worm, not with black blood that is in it.