Oracle Reports Builder - checking if query is empty

I'm using Oracle Report Builder 11.1.2.2.0. I have few queries defined in my report and I want to execute some pl/sql code when there are no rows returned from one of my queries.

So for example:

if (query returned no rows) then
    do_something();
end if;

How can I check it?

You can try to convert your query to a function with exception handling such as

create of replace function get_color( i_color_id color_palette.id%type ) 
                        return color_palette.fg_color%type is    
  o_color  color_palette.fg_color%type;
begin
  select fg_color
    into o_color
    from color_palette
   where id = i_color_id;
  return o_color;
 exception when no_data_found then return null;
end;

and execute the code below

if ( get_color(:id) is null ) then
    paint_it_to_black();
end if;