Anything goes, open up your imagination and creativity.

They can be still-life or the focus of the scene which includes people.  Raw or cooked.  Just make them interesting.

Things that are old, discarded, meaningless or cheap can make an interesting image.  Junkyards, dumps, and other things often have junk.

Let your camera look up or down.  Upward can make the subject seem grand, and downward can make it look vulnerable or insignificant.

Which color doesn’t matter, but there can only be shades of one in the image.  Examples are black and white (that’s two but they are not really colors), and sepia.  But any color will work.

Man-made things. Examples are buildings, bridges, docks.