How is your data presented on the spreadsheet? When I place 7:24 in
cell A1 and 6:00 in cell B1, the result of formula in cell C1, =A1-B1,
is 1:24. I'm not sure why you'd get the series of ########.

As for different fonts or colors when a number is negative, check out
Conditional Formatting. Place the cell pointer cell C1 (in this
example); on the menu click Format Conditional Formatting. In the
window, select "Cell value is" in the first box, "less than" in the
middle box, and enter 0 in the last box. Then press the format button
and select the format you prefer.