If the field name is 'FieldForFormat' you can select all the fields and set their conditional format to be
FieldForFormat = "Yes" [Grey colour]
Thanks but still didn't work. Attached is a screen grab of another field I've set to format to what you suggested. Is that not correct?
Sorry. You didn't say what the field name was that you were basing the condition on, so I used the text 'FieldForFormat'. You should simply substitute the correct field name by picking it from the list in the top left of the formula-building dialogue.