Are those zero values null in your data? Are you using an aggregate function (sum, count, etc) in your expression?
You may need to use the Text Color expression on your expression. Under the Expressions tab, click the plus next to your expression. You will get more properties. Select Text Color and then Edit Expression. Then enter something to color the nulls. It may be:
If(Expression = Null(),RGB(255,0,0))
That will color the null values red, but also pass off any other values to follow your Visual Cues. If you add the false part of the expression, the Visual Cues will be ignored.
It seems like a Sum will usually return zero even if there are null values. Try using:
Sum(ProjectTime + NonProjectTime)
You could also try something like in a new expression:
if(Sum(ProjectTime + NonProjectTime)=Null(), 'Null',
if(Sum(ProjectTime + NonProjectTime)=0, 'Zero',
if(Sum(ProjectTime + NonProjectTime)>0, 'Greater')))
That could tell you what the value is coming out as. You can then use the answer there to create your Text Color expression.