PI is a chromatin binding dye. it is able to bind only with those went through apoptosis or dead cells. While Annexine V is a protein has an affinity to bind PS by a Ca+2 dependent manner. FITC is used as a fluorophore bond to Annexine V.
When cells go under apoptosis ,even in the early stage, cytoplasmic PS turn to the outer side of the membrane a phenomenon enables annexin v to bind with PS.
So, apoptotic cells are going to get green stain (Annexin V) in the membrane and red one (PI) in the nucleus.