Using Adobe:

Using a photo editor when designing new paaterns can help allot with seeing your thoughts before tying them. Start by Changing the Hue on a one color pattern or a two toned with black as one of the colors. Here are some sample results:

After find a color you like you can then change the black using the selective color tool or with some PEs using replace color and come up results like this:

Thus being the simplest of examples but getting the point across. Of course you can edit the nuetrual color to get the grays to the color you would like. Lets try a Master Hue change now, this will change the Hue of R,B,G (Red, Blue, Green)of the entire palette resutling in a total change of all the colors, remember some materials would have to be died to get the results that are shown. A good example in the below picture is the pheasant tippet that forms the tail, I have only seen it in orange the natural color and think it would be quit a challenge to change it.

But for simpler pattern where stuff like marabou is used it can mostly be found in shops or died to the color you want. In the below pictures is a Master Palette change that results in colors that can be found easily in any fly shop.

Now with Adobe and other high end PEs you can select just the color you want to change for example you can take the reds and change only them to blue. This allows you to change just one element or one color in a pattern. Most PE allow the change of Red, Blues, Yellow, Greens, Cyans and Magentas, where Adobe will allow you to use a eye drop tool to select exactly the color you want to change. For instance below I changed the red floss to a light blue then the purple to green and the last the red to a dark blue without disturbing the rest of the pattern.

Also any PE with filters can use a filter named Swap Shop which just swaps the different colors with each other. The below picture I used swapshop, swap B's with R's and G's with R's and so on to get different colors.

Someone can always just invert the pciture which can give some fancy looking picture that woulld be intresting to tie and try.