Why did Strelitzia transform in Episode 15? In episode 15 we can see that after re-uniting with Zero Two, Hiro when connects with her


   Strelizia jumps up in the air to transform into a red version of the FranXX while emitting a bright light, mid-air while transforming  


Why did this sudden change in the FranXX take place?

The only possible reason I can think of is the newly mutual acceptance of each other that made this happen. Are there any canon reasons for it?
There is no real explanation however:


   In ep 15 she accepts herself for what it is after the talk with Hiro. As we know she is originally red skinned.


Further evidence:


   When Klaxosaur Princess used Strelitzia it turned blue as well.