Other than making negative numbers in red, you can also use parentheses for negative number to read them easier to read. Please see the steps below for details:
Step 1: Select cells or range with numbers, either positive or negative;
Step 2: Right click to bring up the dialog box, from the list, click "Format Cells";
Step 3: In Excel 2016 and above, there is no option in "Format Cells" window to apply parentheses with the negative sign for negative numbers, but you can custom with the command "#,##0.00;[Red]-(#,##0.00)";
Step 4: Click "OK" at the bottom: negative numbers turned into RED with parentheses, while positive numbers did not change.