Adjust the Contrast until you can see a change between each cell at the left.  Then readjust your Brightness.  It may take several repeats to get both comfortable.

It's okay if you can't see a difference on the first three shades of black.

These cells have Red turned off.

If the greys have a brownish or other tinge one of your monitors color guns has gotten weak.

These cells have Green turned off.

You should see a rich purple.  If is redish or bluish, you can adjust the monitor colors for clean greys and also watch the results here.

These cells have Blue turned off.

You should see a clean yellow.  If is orange or greenish, adjust the monitor colors for clean greys and also watch the results here.