The popular interpretation of Nolzur's Marvelous Pigments is that a pot can be opened and only a portion of the paint can be used with the remainder being saved for later use. Often discussions of the item specifically mention tracking the amount of paint used. This gave rise to the idea that you could bypass the 25 gold value limitation per painting by painting a single coin worth less than 25g multiple times.
An alternative explanation is that since the pots of pigments are consumable it should be treated as a consumable magic item. A pot of paint is activated and applied to a surface (like oil). The area a pot can cover acts as limitation on the maximum size and is treated similar to over healing on a potion (you cant drink half a potion if you don't need the full amount).
The relevant text from the DMG says:
Some items are used up when they are activated. A potion or an elixir must be swallowed, or an oil applied to the body. The writing vanishes from a scroll when it is read. Once used, a consumable item loses its magic.
The magic item says:
The paint flows from the brush to form the desired object as you concentrate on its image.
Each pot of paint is sufficient to cover 1,000 square feet of a surface, which lets you create inanimate objects or terrain features--such as a door, a pit, flowers, trees, cells, rooms, or weapons-- that are up to 10,000 cubic feet. It takes 10 minutes to cover 100 square feet.
When you complete the painting, the object or terrain feature depicted becomes a real, nonmagical object. Thus, painting a door on a wall creates an actual door that can be opened to whatever is beyond. Painting a pit on a floor creates a real pit, and its depth counts against the total area of objects you create.
Nothing created by the pigments can have a value greater than 25 gp. If you paint an object of greater value (such as a diamond or a pile of gold), the object looks authentic, but close inspection reveals it is made from paste, bone, or some other worthless material.