Hi Jack,
You can set number format as "[Red]$#,##0" to display the value as ($100) in red.
[C#]
//Inserting data into the first cell of the first worksheet.
sheet.Range["A1"].Number=100;
//Applying Number Format
sheet.Range["A1"].NumberFormat ="[Red]$#,##0";
Here is the sample for your reference:
NumbeFormat.zipPlease take a look at the sample above and let me know if you have any other questions.
Regards,
Melba