Yellow is a 'Show alternatives' function for List boxes
List Box Properties - General - Show alternatives
'Toggles whether a selection directly in the active field list box is to exclude the other values of the field or not. If Show Alternatives is checked, all values except the selected value will be displayed as Optional but may become excluded by association with selections in other fields. Otherwise, all other values are shown as excluded by default. This option is not available for calculated fields.'