For some reason, Applejack's Element of Harmony necklace features an orange-colored apple jewel. Dash already has a red lightening jewel, so I guess they wanted to vary the colors, but...it's a bit odd.
Y'know, I always wonder with pictures like this as to why the particular ponies are chosen... I'm sure the answer is simply "It's what I felt like drawing" but Pinkie, AJ and Derpy seems like an odd mix.
The reason I chose those ponies was simply that I was already drawing these panels in my comic at the time. I chose Applejack and Pinkie just because I thought their elements were the most pertinent to the story and what I wanted to write. Derpy was in there simply because casting her as the "Element of Logic and Reason" seemed pretty funny!