Too often, freshly minted MBAs (me included) come to a new marketing role thinking that the way they are going to make their mark is by questioning every little point, color selection, and design decision made by agency creatives. Unfortunately, things all go terribly wrong (for the person nit-picking) when a junior marketer starts picking work apart in meetings. I learned quickly that this was not the way to go, and learning to trust the creatives literally changed my relationships and credibility with our agency partners overnight.
The lesson here is that if you want great creative, trust the creatives to bring it. If you are frustrated with not being able to copywrite every line of that ad for your brand, or you want to change the color just because red is your favorite, quit your job and go become a copywriter or an art director.
In other words, show some respect and you will get what you need.