Can an HP printer I am not using anymore be used for food color imaging?

I would not advise it. If you have used it for regular printing then it will have ink in it somewhere. And that is not good. It would probably mix with the food coloring. For food imaging you really need a brand new printer.

cakes only, I understand the need for a brand new printer, but can you use ANY new printer?

It depends on the type of food coloring paint you are going to use. Some of the food color paint will only work in specific printers. I have a friend that just uses a HP printer that she bought from Wal-Mart. I am not sure where she gets her food color. But she says that she has to get it from one place only.