function ColorToHtml(Col:TColor):String;
var St:String;
begin
St:=IntToHex(Col,6);
Result:='#'+Copy(St,5,2)+Copy(St,3,2)+Copy(St,1,2);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
Label1.Caption:=ColorToHtml(clBlue); //Couleur bleu en #0000FF
end;

function HtmlToColor(Color:String):TColor;
begin
Result:=StringToColor('$'+Copy(Color,6,2)+Copy(Color,4,2) + Copy(Color, 2,2));
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
Label2.Caption:=ColortoString(HtmlToColor('#0000FF')); //#0000FF en couleur bleu (clBlue)
end;