Moss softens an edge
Moss gathers where a surface makes room for it. Against a stone step or an old pot, its green feels less like a decorative choice than a sign of time passing. On a coloring page, several nearby greens and a little untouched paper can suggest its uneven cushion more convincingly than one uniform shade.
Iron gives the garden a skeleton
A thin rail, a gate, a hinge: iron can hold a scene upright. Its darkness makes leaves appear more delicate by contrast. It does not have to be pure black. A hint of blue, green, or warm brown inside the shadow can connect it with the palette around it.
Glass borrows the world
Glass is a fascinating surface because it shows what is beyond it and what is reflected upon it at the same time. In an illustration, a few pale bands or cool shadows can be enough to suggest a pane. Leaving some areas uncolored often does more for glass than filling them carefully.
Wood remembers touch
Old wood brings warmth to the coolness of iron and glass. Grain, softened edges, and marks of repair give a shelf or door a human scale. I like a restrained warm brown beside a greenish gray, with one amber note to imply that the light has found it. Together these four materials make a place feel capable of holding many stories without telling you exactly which story to see.