Consider colors carefully. Too many colors take away from the quick readability of the sign. Again, stay simple. Make sure colors are contrasting. Yellow on white is not readable, whereas black on white is very readable.
Black text is better.
Consistent visual image. Color is probably the easiest and most cost-effective device for this coordination of design for business identification. The proper amount of white space is just as important for quick readability as are graphics, text and colors. Place it to be seen. Your goal should be to make the sign unavoidable to the passing viewer.