yosh64 is basically right: you need to block out basic areas of light and shadow. What will really make your piece look realistic is realistic lighting. This is going to be a sort of long-winded explanation of a basic concept, but it's valuable! We all live on the planet Earth, so we all can see how the sun interacts with things like grass, trees, buildings, and roads. The trick is to translate what we all see around us to your piece of art. So start with the thing that allows us to see at all: the sun!
The sun casts white light for most of the day, and at sunrise and sunset it turns to yellow or orange (depending on how close to nighttime you are). So you should decide what time of day you want your piece to be, and then color your piece accordingly. The sun also scatters its light through the atmosphere, creating blue ambient light that shines from every point in the sky. So begin to place areas of light and areas of shadow: the areas of light should have a lighter white or yellow hue shift, and the areas of shadow can shift towards blue. Remember that you don't just put the color blue in areas of shadow: you should keep the color of the object (for example, a yellow house), but just have it be TINGED blue.
For an illustration of this concept, look at
this reference picture of a yellow house.
Notice where the light is coming from. The white lines represent sunlight. They travel parallel to each other. The blue lines represent the ambient light from the sky. They travel from every point in the sky. Notice how parts of the roof, the porch, the trees all cast shadows on the objects around them. The parts of the house that are facing the sun are almost entirely white with a slight yellow tinge (a combination of a strong white light, a weaker blue light and a yellow house). The parts of the house that are not facing the sun are an olive green (a combination of a weaker blue light and a yellow house). Notice that there are very few parts of the house that
actually look yellow; most of these areas are areas where sunlight is shining on the house with only some of its intensity, because it is partially blocked. Our brain tells us it is a yellow house, so it can be tempting to just use yellow when you are drawing a yellow house. But in order to successfully represent it, you need to get past that mindset and just let your eyes and your true judgment of color do its thing.
You can apply this to any of the objects in your piece. Take the grass. "Is grass really all one shade of green? No, grass can be green where it is well-watered, or light brown where it dies or gets too much sun. How does sunlight interact with the green grass? When green grass is in sunlight, it becomes whiter, but the color also intensifies because the sunlight shines through the blades. In areas of shadow, it becomes darker, grayer, and slightly blue..." ETC., ETC., ETC.
This may sound like a long process, but in your brain you can think it through in a matter of seconds. It's just a matter of choosing the color that you would SEE if you were there, not the color you THINK YOU KNOW is there.
Good luck!