Color blocking means you will be putting all of one color clustered together on your garland (think of a rainbow) and mixed is going to be mixing colors up as you assemble your garland.
Why does it matter?
It is helpful for us to know if you want to color block and in what order you would like them because it helps to know which 16″ large balloons to give you to make sure everything is evenly placed through out the garland. You can always change your mind later and just adjust your assembly pattern a little!
If you are uncertain what you want to do, color blocking is the best thing to put in your personalization box!