Open the report in design view and select tbTotal
Set the Fore Color property to light grey. This will be the default.
On the Format tab of the ribbon, click Conditional Formatting, then New Rule.
Select 'Expression is' from the first dropdown.
In the box next to it, enter the expression [tbPercentage]=1 (since 100% = 1)
Select black from the Font Color dropdown.

Click OK, then OK again.
Regards, Hans Vogelaar (http://www.eileenslounge.com)