Eleve's colour scheme naturally changes in keeping with the Araltian seasons for which she lives. In spring she has a more more natural Leafeon themed colour scheme. In summer her hair turns a more colourful pink, and in Autumn it all turns to reds and browns, before turning to darker colours in the winter.
The funny thing about Araltian seasons to Araltians of course is they can just walk to a different hemisphere without any real problem, so the changes which Eleve undergoes are mostly just an artifact of when her species were significantly smaller than they are today.