Parameterizing the query can solve this problem
hexStr = "0x1234FFCD5";
sql = "Insert into MembersTable(col1,col2,col3,col4) VALUES(@guid, @name, @pass, @hex);";
SqlCommand insertCmd=new SqlCommand(sql, conn);
insertCmd.Parameters.Add("@guid",_GUID);
insertCmd.Parameters.Add("@name",_Name);
insertCmd.Parameters.Add("@pass",_Pass);
insertCmd.Parameters.Add("@hex",hexStr);
insetCmd.ExecuteNonQuery();
If problem persists, please provide the full code.