art supplies; art materials
Refers to physical materials used for drawing or painting, such as paints, brushes, canvas, etc.
I bought new paints at the art supply store.
subject matter of a painting; motif
Refers to the theme or subject depicted in a painting, not the physical materials. Less common than the 'art supplies' sense.
この画材は初心者にも使いやすい。
These art materials are easy to use even for beginners.
The subject matter of his works is mainly landscapes.
art supply store
Compound of 画 (picture, painting) and 材 (material, subject). The 'art supplies' sense is the most common modern usage.