When creating a formula, ThoughtSpot provides suggestions as you type, in much the same way as when you create a new search. These suggestions consist of columns, functions, and sometimes syntax.
Column suggestions are shown in purple, and are usually followed by their source information. Function suggestions, on the other hand, are shown in blue.
So, in your example, the purple date is a column from your source table, while the blue date is a function. The date function returns the date portion of a given date.
You can find more information on different functions by clicking the Formula Assistant in the formula builder.