procedure TForm4.ComboBox1Exit(Sender: TObject);
var ok:boolean;
begin

ok:=false;
if upcase(combobox1.text) = 'YES' then ok:=true;

if upcase(combobox1.text) = 'NO' then ok:=true;

if ok=false then combobox1.Color:= clLime
{if any of the above in incorrect then make the colour lime green to show error}
else if length(combobox1.Text)=0 then combobox1.Color:=clred
{if there is nothing there, the box goes red}
else combobox1.color:=clWhite;
{otherwise it remains white}
end;

end.

procedure TForm4.ComboBox1Exit(Sender: TObject);
: var ok:boolean;
: begin
: 
: ok:=false;
: if upcase(combobox1.text) = 'YES' then ok:=true;
: 
: if upcase(combobox1.text) = 'NO' then ok:=true;
: 
: if ok=false then combobox1.Color:= clLime
: {if any of the above in incorrect then make the colour lime green to show error}
: else if length(combobox1.Text)=0 then combobox1.Color:=clred
: {if there is nothing there, the box goes red}
: else combobox1.color:=clWhite;
: {otherwise it remains white}
: end;
: 
: end.