To insert a colon into a column of existing numbers, follow these steps:

1. Insert a new column next to the column of numbers you want to format.
2. In the first cell of the new column, enter the formula
   Code:

   =LEFT(A1,LEN(A1)-2)&":"&RIGHT(A1,2)

   (assuming your original numbers are in column A). This formula will take the left part of the number (everything except the last two digits), add a colon, and then add the right part of the number (the last two digits).
3. Copy the formula down to the rest of the cells in the new column.
4. Select the new column and copy it.
5. Right-click on the original column of numbers and select "Paste Special".
6. In the "Paste Special" dialog box, select "Values" and click "OK". This will replace the original numbers with the formatted numbers.

That's it! Now you should have a column of numbers with colons inserted between the hours and minutes.