The six-color beginning
For a greenhouse scene, I might lay out a deep moss green, a lighter sage, muted mauve, smoky teal, warm amber, and a soft brown. This is a mood board rather than a shopping list. Your tools may have different names for these colors, and the exact shade matters less than the relationships between them.
Give the light a job
Use moss and sage for most of the foliage, varying pressure so the leaves do not become a single block. Let mauve visit a petal and one quiet shadow. Teal can live in the glass or a ceramic pot. Save the amber for a lantern, a lit pane, or one reflection on the path. Scarcity helps the warm light feel warm.
Hold back some paper
A pale pane or a moonflower does not need to be colored all the way to its edge. Leave a little white to suggest brightness. If the ironwork feels too stark, layer a small amount of smoky teal or brown around it rather than filling every bar with black. Step back occasionally and notice where your eye goes.
Make a second version
Now change one thing. Replace amber with a cool lilac, or let the mauve become a rusty coral. The same line drawing begins to tell a different story. That is the pleasure of a palette: not proving there is one correct answer, but discovering how a few deliberate choices can open a new world.