Believe it or not I was the first to tie with different color's for a Parachute post. And yes I can prove it. I worked for a large fly fishing company as there manager of flies and materials. They still have some on my flies in there cataloge with the colored posts on them that I tied. I would get in new materils and use them tying different fly pattern's to see how they worked. That is how I came up with using the different colors on the posts. Now I was not the first to come up with the Parachute post or the color white for a post.

Here is how I use the different colors and why. First I use a black post in flat light such as over cast days or after the sun has set. Try it, it will amaze you.
If you are in foamy water use orange or hot pink. You will pick it up better than white. I think you can understand that one LOL
The rest of the colors are just what you would like to use. But I carry at least three different colors on my flies. Ron.