The most important element to remember when making individual color choices while painting is the temperature (color bias) of the light. The light that illuminates our scene is the uniting force. Everything shares one common thread—light! By learning to analyze the bias or influence it's making on the base colors, we can unite our painting and end up with a finished painting that has harmony. Our mind will associate individual colors to the things it sees: a tree is a certain green; a building a specific red and so on. What is overlooked is the color of the light and how that influences everything it illuminates. Tie the individual colors together with the light and you’ll end up with a harmonious painting.
 
A great exercise to help us see the color bias of the light is to place a white board outside your home in an area where it will receive a moderate amount of light throughout the day. Observe it at different times during the day and make note of the color shift of the white. You may wish to study this in open shade as well, as a way of comparing those differences. This study can even become a seasonal exercise in analyzing how light changes over the course of the year. By doing this, you'll better perceive color shifts within the light and how it affects the objects it illuminates.
  
Applying what you observe to your pastel work is a matter of choosing colors that lean toward the color of the light. Late in the day, for example, the color of the light may have an orange bias. This would make a warmer, slightly olive green a better choice for the tree foliage, and accordingly  warmer choices for the other elements within the painting. A method I often use to tie the color of the light into every area of the painting is to pre-select a range of values around the color I feel the light is emitting. If it's a soft amber, then I will select five or six pastels in values from dark to light to represent the light, adding a little of the appropriate value to each area of the painting. By staying true to the value of the individual areas, the form is retained and just the color is being shifted towards the temperature of the light. In Last Light on the Water-lands (pastel 12x16), for example, an early evening light created a warm cast to the scene; a little bit of "amber-colored pastel" was added throughout the painting.
 
By spending time educating ourselves to see the color of the light and employing methods like a range of values in the color of the light, you'll develop a keen ability to analyze the light and intuitively select the appropriate pastels, thus creating a harmonious painting where everything appears to belong together.