Hi!
Please have a look at our comprehensive diet guide, which looks at all food groups in detail. While piggies can eat a wide range of veg not everything is equally good for them. Carrot for instance is like eating a bar of chocolate for guinea pigs - no wonder do most piggies like it!
Guinea pigs learn from their elders what is safe to eat and what not in the nursing period and the formative 'school weeks' in between weaning and teenage during which they learn to master their environment and the intricacies of their complex social interaction by shadowing elders and picking foods from their mouths. Like all rodents, guinea pigs don't have a vomit reflex; so what has gone down has to come out at the other end. They have however double the number of taste buds of humans and five times as many as cats to help them to not swallow anything that doesn't taste safe/right.
If your piggies come from a place that offers no veg or only a very limited selection, then introducing a wider variety can be hard work and takes patience and persistence because as only piggies in a home they lack the piggy guide that they'd normally imitate and follow.
Here is our diet guide:
Long Term Balanced General And Special Needs Guinea Pig Diets