Thank you for your reply, it was a problem in the expression I think (the and condition). When I changed the expression it works.
Expression :
=Switch(((Fields!myField1.Value) is nothing and (Fields!myField2.Value) > 0), "Red", ((Fields!myField2.Value) > (Fields!myField1.Value)), "Yellow",
(Fields!myField2.Value) < (Fields!myField1.Value), "#ff7f00",
(Fields!myField2.Value) = (Fields!myField1.Value),"White")
Regards,
Anis