Automatic change of background color in Excel

This looks like a conditional formatting rule that highlights duplicate values. That is why choosing No Fill does not work: conditional formatting overrides the cell’s normal fill color.

Select the affected cells, then go to Home > Conditional Formatting > Manage Rules. Set Show formatting rules for to This Worksheet, find the Duplicate Values rule, and either:

• Delete the rule if you do not need duplicate highlighting, or
• Edit its Applies to range so it affects only the intended cells.

You can also select the affected range and choose Home > Conditional Formatting > Clear Rules > Clear Rules from Selected Cells.

After removing the rule, apply No Fill again if any background color remains.