Downtown Lumiose city, there is a salon known as 'Ego On The Outside', run by a lone scientist named Ernest. After paying 10,000 PD he will put your Pokemon inside the machine and transform their outward appearance to what matches their personality.

Use this like the shop.
Recolour a Pokemon sprite, pay the 10,000 PD and your Pokemon will look like that in the RP. Post the sprite here when you pay.
It costs another 10,000 to change their appearance again.

If you wish to change your Pokemon back to normal it will cost 5,000 PD.

Maddy and Dark can recolour sprites for you if you can't do it yourself.