Derek notices this and tries to figure out why this bothers him. Meredith has worn brown before, but why can't he stop thinking about it now? He is in an on-call room, between surgeries, so he has a good chunk of time to ponder the colour of Meredith's sweater.
And then, after twenty minutes, he has a revelation.
Brown is the colour of dead leaves. The ones that were once green and vibrant, but the surroundings have forced them to become lifeless. They fall from the trees, floating downward, eventually hitting the ground. Maybe being trodden on by people who don't mean to do so, but break the leaves into a million little pieces with every step.
This doesn't need to happen to the leaves, though.
The wind could pick them up, carry them off to a safer place. Maybe a happier place. The wind can help the leaves.
And Derek wishes, more than anything, that Meredith would let him be her wind.