The dry feed should not contain chemical colourants as they can give symptoms of diabetes (very high blood sugar readings) amongst other abnormalities. Many of the popular food brands are the equivalent of junk food for your guinea pig and just as you would not thrive on eating fast food for every meal, neither will your guinea pig. The ingredients list has to, by law, state whether this chemical is included. Once you have found a supplier of chemical colourant free food you will need to check each purchase as company’s sometimes change suppliers which could mean the addition of chemical colourants.
We supply a chemical free dry feed. We would never supply our guinea pig’s or yours with anything which is not first approved by us and second, free from chemical colourants. At the time of writing, they are Wagg Guinea Pig Crunch, Wagg Optimum, Burgess Excel and Pets at Home Museli.
If you buy your guinea pigs dry feed loose from a bin, you need to know if the chemical colourants are included from the analysis on the back of the bag that the feed was in, before being tipped into the bin. I would be very sceptical about buying dry feed if the analysis is unknown. The bin and scoop should also be clean and dry. It should not have feed stuck to the sides indicating old feed or damp which probably harbours bacteria.