What color are you thinking to paint your kitchen? And I also assume you want to keep the country blue you have in there now and not change it at all.
You could paint the common wall with either of the two colors, if it coordinates and can work with the other room's decor/color scheme. Another thought is to paint that shared wall a neutral color or a color that compliments BOTH rooms. If the colors of the two rooms work well together, you can put a chair rail on that wall and paint the lower half of the wall one color, upper half another.
Some other methods to create a dividing 'line': It is possible to put a piece of molding vertically to divide the wall/distinguish the room - then at the top (where the wall meets the ceiling) use a metal decorative 'bracket' to sort of create a doorway. Here's a link to a bracket so you can see what I'm talking about:
http://www.ballarddesigns.com/Wall-Decor/Brackets-Hooks/Acanthus-Corner-Brackets/p/2675?path=1%2C2%2C1468%2C1536&iProductID=2675
Another option is create a fake window - hang floor to ceiling drapes (or fabric) in front of the wall to hide the line where you go from one color to another.
You can also take wood and paint it a neutral color, make it a reasonable width (think 6-12") and put that in (floor to ceiling) then write sayings on it or use it as something your guests can write messages on, etc. It may not work for your taste/decor, but it's certainly whimsical and brings your family/friends in on things and it's something that will bring a smile to your face when you read it. (You can also use it as a growth chart for kids.)
A floor to ceiling board covered in batting/fabric/ribbon, to create a french memo board type thing - tuck in photos, notes, cards, etc. You can frame the board and that decorate it how you choose. Check out the below links for a visual. Obviously, you'd have to make your own and do it floor to ceiling, but it's a great option. Would look great empty or with your photos/cards, etc.
http://store.wellappointedhouse.com/memoboards.html
http://www.amazon.com/exec/obidos/search-handle-url/index=blended&field-keywords=fabric%20memo%20board&results-process=default&dispatch=search/ref=pd_sl_ya_tops-1_blended_41227399_1&results-process=default
Best wishes - you have lots of fabulous options and hope more members of the community have even more to offer! I'm sure you'll find the perfect one to make it work and pull it all together!