The yellow and red was nice but as you said, too much details in comparison of the whole icons.
Of course the pink is Kaylee's colour ! strawberry, her dress… I don't know how I missed that.
For shepherd Book, a Bible could be more obvious : "River, you can't fix the Bible, it fixes you."
The Glasses for Simon are too anecdotic, I don't remember wearing them after the pilot Serenity. Instead I suggest a syringe or a surgery knife, something medical that he's almost the one to use.
For Zoe, I'm not convinced about the iron knuckles but I have nothing better in mind