This is the common practice to waste time by native English "teachers"( especially native speakers with no teaching experience) who get paid by the hour. The other method is to pick some words from a pop song and concoct some meanings for them under the guise of "getting the feel" of Western culture!!

I can tell you that artists know what a 'warm' yellow means.  It's not all that subjective, and its not adverbial. It is not 'in what way' is it yellow, rather, it is 'what characteristic of' yellow.  The name of the color, when it has a particular characteristic applied to it, may be in many cases a noun--as Eimai Anglos says. 'It is a dark yellow.'  That is a sickly yellow.'  'Van Gogh is known for his intense yellows'. 

But there is also:  A brassy yellow pigment .  A dusty pink dress. 

I keep thinking about this 'warm yellow' question. 

Experienced forum members have seemed to agree with CJ that this is a 'uselessly baffling puzzle'.  I am curious about the reason, maybe because it has been a long time since I approached writing and speaking English from a conscious memory of grammatical terminology,  I had to look it up to re-learn that adjectives do not have as one of their official functions the modification of other adjectives. 

And yet, it seems to me here, that unnecessarily convoluted thinking is required to demonstrate how 'warm' is not an adjective modifying another adjective, 'yellow'.

Would the problem in 'a brassy yellow pigment' go away if 'brassy yellow' were hyphenated, 'brassy-yellow'?  In 'the dusty pink dress'--if it's not the dress that is dusty, should it be written, 'dusty-pink dress'?

My puzzling over this probably stems from my interests as an artist.  If a definitive anwer is not forthcoming, I will consider my bafflement useful.