Most pet shops sell two kinds of food
Museli - the coloured stuff, rabbits tend to selectivley feed on this (meaning they eat the bits bad for them and leave the good bits) Generally not very good for your bunny. Common brands are Russel Rabbit and Wagg!
Pellets - you can get a range of pellets, mainly brown and all look the same, and stop selective feeding. Alot better for your bunny
Common brands are Pets at Home Nuggets, Science Selective, Excel Alan and Paige,Wagg(sometimes you may have to get your petshop to order anything specific
Also, youll probably be given some of the food the rabbit is currently on.
If your rabbit is on museli,and you want it on pellets, this has to be a gradual change over as a sudden change can cause tummy upsets
Personally, I use both feeds. My 2 outdoor are on Excel pellets and my 2 indoors are on Museli mix (mainly due to problems with getting them to change over)