John.
are your range of numbers set to bebetween 1 - 100?
you can accomplish this using conditional formating with 2 rules.
First look for duplicates in your data and in a second row or over several rows where you list all numbers between 1-100 like Harry did in his example.
second look for duplicate in your original data using a different color.
missing duplicate.zip
hope this gives you another way to accomplish what you are going for.
Don