It is important to choose colors for your sign that have a lot of contrast. Certain color combinations contrast more than others. For example Red & Blue and Red & Green do not contrast well. You probably would not want to create a sign with a blue background and red print. You may be able to see what it says up close, but from a distance you probably won't be able to read it at all.
Shades of the same color also do not typically work well. For example if you want a darker orange background with lighter orange print. You won't be able to see the letters well at all.
You want the artwork on your sign to "pop" out so that it is easily visible from near or far. The more the background and print colors contrast the more likely this is to happen.