I finally found a solution.
The example for "Using Formulas" induced me to error.
If I use .WhenEquals() doesn't work, I have to use .WhenIsTrue()
Here is how I color a row from A to N depending on the value of M column:
ws.Range("A1:N100").AddConditionalFormat().WhenIsTrue("=$M1=\"Hello\"").Fill.SetBackgroundColor(XLColor.Gray);
And here is how it is shown on Excel:
It works correctly.
Thanks.
