There has already been a lot of discussion about dull colours when exporting to pdf. If you go to the top of the main forum page and type a small description of your question in the search box, you will often find that question has already been discussed. Here is a link to one of the more recent discussions: http://coreldraw.com/forums/p/7092/32023.aspx#32023 I typed 'pdf colors dull' in the search box.
Once you have read the above discussion try doing another search to see if there are other discussions. I suspect members of this forum will not want to discuss the same subject again and again so you may not get an answer, so give it a try.