Conditional Formatting of Blank Values in Matrix using DAX

Hello,

I have a matrix with working times of users, where rows represent invidividual users and columns are dates. Cells represent users working time in a specific day. I want to highlight all cells with value less than 8 and all blank cells with red colour. I need to do this by creating a measure using DAX and not the rules available in conditional formatting. Example data look like this:

UserTable:

DateTable:

HoursTable holding the actual data:

In the model, the UserTable.id is connected to HoursTable.userID and DateTable.Date is connected to HoursTable.Date. It looks like this:

In the HoursTable, I've created a measure Colouring that determines the colour of each cell in a matrix. If the value in cell is less then 8, assign red colour. If it is black, assign red colour. Otherwise, assign no colour.

I've created a Matrix visual using my data

And assigned conditional background colour formatting based on the measure value

As a result, I got a Matrix where cells with value less than 8 have red background. However, the blank values have no colour:

However, if I add the Colouring measure to the matrix as an additional value to be displayed

Then the background of blank cells is suddenly coloured in red

Naturally, this is not desirable because the Colouring measure should not be part of the Matrix values

What am I missing here? Why is it behaving like this and the background is coloured properly only when I add the Colouring measure to the Matrix visual? How should I create a DAX measure or modify my data model to achieve what I desire?