Is her dress blood red? Or rose red? Do the stars twinkle like 4th of July sparklers? Or blink like a million ogling eyes?
Use the senses to set the mood. Two characters on the beach begin to kiss. How do things smell, taste, feel, sound? Remember, you're establishing an atmosphere.
Does John nuzzle Allana's neck, breathing in her lilac perfume, then kiss the salty sweat at her hairline? Does he feather his fingers along her arm, drawing goose bumps?
Does John nuzzle Allana's neck, breathing in her bologna breath as she sighs, then spits hair and sand as he tries to kiss her earlobe? Does he go to caress her arm, but rams his elbow on her hair, yanking her head to one side and spilling the pitcher of ice tea across his sunburned back?
A woman has had a long, arduous day at work. She draws a bath, pours in foaming oil. She touches the bubbles as they build. They're soft, like whip cream. She steps into the tub and slides down until the bubbles tickle her chin. How does the bath smell? Cherry? What kind of cherry? Is it a cherry-Coke float? Cherry cough syrup? Cherry sour balls eaten in the darkness of the movie theater? Grandma's hot cherry pie?Each one brings up a different image, sets a different mood.
Choose your words, your sentence construction, your details so that they set a mood. Each scene has an atmosphere.
This is not to say that if your book is meant to be humorous, then every scene must be funny. There will be an ebb and flow. You don't want your novel to be monochromatic. But all the scenes together establish the overall mood of the book. Use your words--you are a writer, after all--to create the atmosphere of your book's world.
Do you have an example of how you used words to set the mood?