I take it that they are from the mixed up cutie mark episode. Seeing as Pinkie has AJ's mark and AJ has Rarity's mark. That or Applejack and Rarity are doing some serious role play. Which if that was the case, it would be easier for Rarity to be Pinkie and leave AJ as AJ since she knows the part well. Though I can see Rarity not wanting to be Pinkie as she is to chaotic. So they went with these roles. Though Cutie Marks are magic. Thus why they shined through the Orange and Pink (I might think edible as well) body paint.
Then again I may be giving this to much thought. Great picture.