Report Builder - Background color expression and blank/null values help

[TKeys8]

So, I'll try to keep this brief - I'm creating a paginated report and I'm trying to make the background colors unique per certain conditions. I have gotten this to mostly work. 

For example, the "Status" column I have works perfectly against every other column. While every other row is alternated in color, the "Status" conditions have their own colors that override the alternating colors. 

The issue is with a "Labor Difference" colomn. Here, the value is calculated as the difference between the quoted labor and the actual labor performed. I would like to have the background color change so that if a labor difference is less than zero, the cell becomes red. 

This should be simple, but it's given me an issue that I have not yet been able to solve. The red coloring works perfectly (and I've seen it update live since I'm using a live data set - the numbers keep changing!), but then... all the color formatting goes out the window when it has no values to work with. It has no values when the "Actual Labor" hasn't been entered, so it is a blank/null and therefore displays as empty. 

However, as I mentioned, the formatting goes out the window... for just this column. Blank values in the adjectend columns are still colored properly, meanwhile the blank values in the Labor Difference column are the same color as the main background. See the attached image for a visual of what I mean. 

I would absolutely love to just write an expression that utilizes nulls and/or blanks, but I haven't been able to figure out how to do that. Any advice would be lovely!