Step 1: Select the 2 columns

This step is not difficult but there is a trick for selecting the 2 columns.

1. Select the range of cells in the first column
2. Press the Ctrl key
3. Select the range of cells in the second column

Step 2: Open the conditional formatting tool

1. Go to the menu Home > Conditionnal formatting
2. Highlight Cells Rules
3. Duplicates (yes it looks crazy but it's the menu to select)

Step 3: Select the Unique option

Here is the trick! In the dialog box, you select in the dropdown list, the option Unique

Immediately, the missing values between the 2 columns are in red

Use a formula to highlight the missing values

As you can see, with this technique you necessarily colored the missing values in the 2 columns. But if you want to find the missing values for only one column, you must use a formula.