It’s the cement between the bricks in writing. Where are you? The sky, colours, shadows across a building at dusk.
If your story includes a broken window. Describe the damage. A cobweb of glass. The bits of wooden window frame scattered on the green carpet.
The great French impressionist painters used to sit in parks or wine houses sketching individuals. Those individuals were then incorporated in to a larger work for background and colour. Be like those painters. Use the same trick.
A guy jogging down the road listening to his music. Or a flower seller leaning against a wall. Or two people kissing on the pavement. May seem mundane of themselves. But in a story where the cops are cruising the streets looking for the lost child. The jogger and the flower seller and the couple will give the work some colour. Which reminds me to tell you.
Always, always carry something with you that you can jot down an impromptu moment. Don’t tell yourself that you’ll remember it later. Because chances are YOU WON’T!
One other tip on this subject. NEVER,NEVER, NEVER throw anything away if its image notes. Or a piece of writing that on reflection you’re dissatisfied with. Because I can guarantee you the day after the dustman has taken it away. You’ll realise that you could have used that repartee, that image, that description of the lady with the gold watch for something you’re currently in to.