This optical illusion is due to the striped bars that surround the hearts, the orange hearts have green bars around them, whereas the purple hearts have blue bars; a little trick that our brain delights in.Check out this video below from the creator,
Bite Size Psych, for the full explanation and for another couple of illusions that our brains trick us with.
It just goes to show us that our brains and visual receptors don't only depend on fact but are influenced by context too.