Everything being the same color can easily lead to a feeling of being "in a maze of twisty passages, all alike". It's like gray dialog boxes that pop up more gray dialog boxes: technical users tend to have an ontological hierarchy in mind as they navigate so it's no big deal, but most people get quickly overwhelmed and feel lost/overwhelmed. [Not that I'm saying that simply coloring dialog boxes differently would necessarily help here! Just that them all being gray adds to the overwhelm.]
I strongly disagree that color aiding in use points to a larger accessibility problem — perhaps you just mean that if color is necessary for use you've got a problem (which I'd certainly agree with), but while I could get by in my everyday world with monochrome vision, I'm glad I have color cues all around me that aid me in distinguishing amongst objects quickly and with minimal effort.
Yes, you are right. Color when used appropriately can improve the use of a UI. But, he/she implied that it was unusable simply because the icons lacked color. That is completely wrong.
I strongly disagree that color aiding in use points to a larger accessibility problem — perhaps you just mean that if color is necessary for use you've got a problem (which I'd certainly agree with), but while I could get by in my everyday world with monochrome vision, I'm glad I have color cues all around me that aid me in distinguishing amongst objects quickly and with minimal effort.