I would like to ask if the words "Grain Honey Nut" in "Grain Honey Nut Cheerios" are adjectives or not. If not please can someone tell me what word class it is?
In a similar way, I would parse the two word phrase "Honey Nut" as a two-word adjective.
So, all told, I'd parse the name of the cereal like this:
adj adj noun
[Whole Grain] [Honey Nut] [Cheerios]
Incidentally, when a word that is usually a noun functions as an adjective (as do chicken and paper in the phrases chicken soup or paper clip), this is knows as a noun adjunct.