Running balance into minus figures

Evening everyone.
Firstly I just want to say - fantastic forum, I have found many answers here that have helped me along the years, but now I need some more specific help.

So I am creating a running balance on a sheet in 2003 using hh:mm custom format. What I want is the cells to calculate a balance into minus figures. The positive figures show as green, the negatives as red, I used conditional formatting for this. But when the balance requires a negative figure the display is ############

Getting a "-" to display in excel is always a bit of a pain for me, I just enter a ' before and this normally works. I had to use that technique to get the conditional formatting to work properly in this case. I was thinking of using a custom format along the lines of hh:mm;[green]0.00;[red]-0.00; but I have had a play around with that and no luck.

This has really got me stuck and any help would be greatly appreciated as always. I have attached a draft of the workbook I am using if it helps.