The liquid is hydrogen peroxide used to lighten them whilst in the sun. However you have to be careful with ponies that have rainbow hair in that solution since it could leak and stain the pony's body!
Theres some info about it on FarDreamer's website MLP preservation project under 'body'
Ive never tried it myself but heard it can do wonders as can acne cream too with certain things in it and with supervision of course!
Hope this helps you