It is one thing to work with color when you have a broad range of it, but it is another thing to work only with the existing colors you have. I would occasionally go and get one skein of yarn to use with what I had but for the most part I worked only with what was present in view.
With these two items for example, the mitered bag and the afghan was made all with scraps and I had one skein of yarn for the sides, bottom and straps for the bag. But I ran out of the grey so I had to work with another shade of grey to finish the straps but it all blended in.
Working with what you have is a challenge. I had to put colors side by side and rearrange them to see just what would go together. I had to visualize how often I wanted to use a color, how many different colors I would use and in what order the colors would be placed. Your vision is sharpened and your thought process is also. And wala you have your masterpiece.
There are different ways of being creative. One is opening up possibilities of working with what you have where that in a circumstance of buying to match color doesn't give the mind the same amount of creativity. So it forces your mind to be more creative and to see what you might normally not see if you just use your freedom to buy.
So the next time you feel you just need to go out and get more yarn, think first as to what you already have and sharpen your color skills and learn to work with what you have and challenge yourself and see if you can come up with your own color masterpiece.