ORACLE - How replace NULL value

nvl() function replace NULL value with second parameter (..in this example 0):

function sql_Color_AddNew_Rule( pQuestion varchar2, pUser varchar2, pGroup number ) return number
is
  iID number;
begin

  /* -- find new ID */

  select nvl( max( c_id ), 0 ) + 1 into iID
  from factory.an_color
  where
  lower( c_question ) = lower( pQuestion ) and
  lower( c_user ) = lower( pUser ) and
  c_group = pGroup;
...