This is offered from an artist's point of view, and I do hope it is of interest and help to someone. I do not have photoshop so take from it what you will.
If you have flat 2 dimensional object, like a painting on a flat surface, or a photograph you are colouring, and you want to make it look more three dimensional, or more real, there are some ways to make that happen.
Think of what you see when you look at a real landscape with a far distance.
As objects retreat into space they become less distinct of course, and smaller, but also the colours appear more grey and less intense.
This intensity is called 'value'. An example would be, if you were looking at a range of distant mountains and some closer overlapping forms, such as trees, mountains, houses, rocks, clouds, people, etc anything appearing closer to you, will be more intense in colour. The distant mountains will look pale, misty grey blue and would get more pale ( less intense) and greyer the further away they are.
Trees in the distance are not as intense in colour or in detail as are closer ones. An artist will 'grey down' the colour on the palette with white or some other method, to paint the distant trees, to lessen the value, to visually imply distance. The further away the object, more indistinct is the colour and the detail.
As well as value or intensity of colour, using warm and cool colours can imply three dimension and depth. There is a 'cooling' of colour as objects appear further into the distance.
Warm colours are, generally speaking, those such as, yellow, orange, red, warm purples, ect. They colours of fire.
The cool colors are cold whites, blue-greys blue, blue-green blue-violets etc. violets., or Winter colours.
On a flat surface, if you want to make your picture look as if there is depth and dimension, use warm colours in the foreground and cooler colours in the background.
The effects of colour should
gradually change from intense to less intense, and from warm to cool as they move into the implied distance.
Remember this is 'generally speaking' as it takes a lot of experimenting to get it right, and it has to look right to you.
For instance, there can be a 'cool violet' and a 'warm violet'. one is more blue and one is more red. There can be a 'cool green' and a 'warm green.'
Cool down a green with a little blue, and you have a cool green, add more yellow and maybe a touch or orange, and you have a warm green, suitable for a foreground.
It's a sort of trickery of the eye.
So by using both techniques together, such as more intensity of colour and warmer colours in the objects you want to appear closer, and using less distinct, greyer and cooler colours in the objects you want to recede into the background, you will create an impression of depth.
There are three basic colours red, yellow, and blue. How you mix them will give you a lot of variations. Red and yellow make orange, blue and yellow, make green, blue and red make violet. adding white to any of these or making the colour thin and transparent will lessen he intensity. Theres is alot more to colour theory, but those are the basics.
Also if using greens for outdoors, not all trees and grass are the same green. Look around you, look really carefully, look at close objects and look at distant objects to see how colour changes.
Don't think tree trunks are supposed to be brown because your kindergarten teacher once told you that. What colour are they really? Go for a walk and look at them. Some of them might surprise you.
