Background "things": Felt squares, cloth, colored papers, embossed foils, textures papers ...

Translucent "things":, artist/draftsmen vellum, etha-foam (packing foam), translucent condiment cups, styrofoam (**) cups ...

Reflective "things": mirrors, glass, foils, true white cards/paper ...

Opaque "things": paper, plastic, foam board

True GRAY "things": Paint sample chip from Home Depot or Lowes