You may get some answers here but I would cross post to the decorating forum too.

I have a similar space but I have archways to my family room, eating area and 'sunroom' so I have natural breaks. I have wood flooring in the kitchen and eating area but kept the carpeting for now in the family room and sunroom, that helps define the space (and was budget friendly.) No pics, still a work in progress.

Will you have a beam between the two or is it completely open, no defining architecture between them? What other spaces are open to the kitchen? What kind of light do you have in each room?

On the advice of our painter, we ended up using the same color in all the space that was open to the kitchen; right now that includes the LR and the hall to the bedrooms. We used full strength in the kitchen and cut it by 50% for the LR and hall, but you can't tell the difference, even where the two shades meet directly. If you are using a light color and it works for both rooms, I think this will be fine. If you have a darker color, you may want to consider a lighter color on the same strip for one of the two rooms, maybe the one that gets less light.

It's very hard to judge the color in these photos. The front wall with the windows and the front door is white; we didn't paint it since we'll be adding forward this fall. But these photos should give you an idea of how the two rooms flow together.

I have the same layout..We haven't painted yet so I don't have any useful pictures to show right now. What I can tell you is I plan to do what jsweenc mentioned above. I am going to use two different colors, but on the same swatch, just one a lighter version of the other.. I haven't decided completely yet what colors i am using, but that is my plan right now..

Thanks for the replies and the photos. Both rooms have a lot of natural light. There are doorways into other rooms, but nothing else really "open" to the kitchen. There is no defining architecture between the two rooms right now and I was considering having a beam wrapped in trim installed on the ceiling. I wasn't sure what to do with the walls though. One wall has a natural break, but the beam on the other wall would fall to the right of a double patio door. Jsweenc, it looks as though your beam falls right over the door trim. Ours would be to the right, too far away for the trim to act as a break between the two rooms. By the way, I love the windows with no upper wall cabinets. We are doing the same thing!

You can add a strip of molding on the wall vertically where the paint can change between rooms.

In our last house it was all open plan and I had a heck of a time figuring out where to change the kitchen to living room color as it had a lot of angles and breaks in the ceilings and yet the wall went straight along the whole front of our house with no break or corner anywhere. Once it was done, it was never an issue and I bet no one would have noticed whichever way I did it. You can always try one way and rework if it does not look right.
I agree about making it along the same color family in different shades or making it an equal shade (like one light color to another, or a medium to another). I don't think you'd want great contrast from one to another.
We are making it open plan on this house and will have a light lavender in the kitchen, to a light green color on the back wall to a darker green color on the next wall that is 2 steps from back wall. The front wall will end up the same light green as the back. This way each transition is smooth and yet we won't have it be monochromatic. The space is too big for one color and it will cover 4 separate function rooms (kitchen, den, dr & lr). Some designers would do all one color for drama, but I love to pick paint colors and mix it up, so that is not an option for me. ;) I can also change a wall or 2 without having to do the whole space if I get bored.

Dianalo, I was thinking of doing something like vertical trim or molding on the wall. If I did that on one wall, would I have to do the same on the opposite wall for balance? How did you handle the change in colors in your last house? Do you happen to have any photos to share? Thanks.

What kind of colors are you considering? That might make a difference.

Yes, that beam was an issue. We decided to take the trim over that door all the way up because I hadn't decided paint colors yet; I wanted to keep my options open, but in the end, the other end of the kitchen going into the hallway didn't have a natural break so there was no way to avoid it. With the color we chose (Quiet Moments by BM) I think it works fine; in fact I love it. It really lightens up the two rooms and the hallway. (We have a sconce at the very end of the hall that I was sick of seeing with the two-toned green paint that was in the hall before and was ready to get rid of it; with the new paint, it's like I have a new light; I love it now.)

The beam may provide a place to do as dianalo suggests, to run a piece of molding all the way up.

I forgot about the back hallway. I wanted most of the utility room to be a medium brown but the hall to the garage has no windows (what you see is coming through the open door from the adjacent room) so I wanted it a lighter color. I had it painted two ticks up on the strip. (Darker is Safari Beige and lighter is Oat Bran by Valspar.) I know you're talking about transition along the same plane, but I thought it might help to see two shades right next to each other.

I can't wait to see what you're doing! Do you have any pictures yet?

We did the same thing, we ripped down the 1/2 walls between the rooms. I love it. The walls really just got in the way in my opinion, I had both rooms painted the same color before. I think that by painting the rooms the same color is what gives you the feel of the "Great Room", you can use an accent color on maybe a facing wall? Here are some pictures of my space (sorry, if you are tired of seeing them), I actually am in the process of choosing a new color--it just needs to be tweaked--the gray is a bit greenish for my liking.

Actually, the clock wall in the photo is a shade darker than the other walls, and the fireplace wall is 2 shades darker.

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I will have to take a photo of my kitchen-family room but we have the same paint in the kitchen and family per the advice of a decorator. We have a fireplace on the far wall of the family room so we have a accent color on either side of the fireplace. The family room ceiling is vaulted so even though the paint is the same color, it looks different in that room. People are always asking us if it is the same color - I think the subtle lighting difference looks really nice. We considered putting a vertical strip of moulding to define the space but the decorator discouraged us.

Thanks for the additional photos. It does help to see the different shades next to each other. I'm doing a white kitchen and was considering white walls, but don't want to use white in the family room. I'd like to use a warmer color there. I don't have photos yet, right now it's gutted down to the studs.

Beekeeperswife - I love your kitchen (and backsplash) and don't get tired of looking at the photos. My rooms have virtually the same layout as yours, except reversed. My kitchen is on the right when facing the outside wall. I'll have L-shaped cabinets and a narrow island and have the fireplace on the end wall. What size is your island, by the way?

Ok... now I feel silly. I was trying to remember how I solved the dilemma in the last house and could only remember struggling with the decision, but not what I had decided.

I found this old photo and it seems I ended the kitchen color (lavender) sooner than I had remembered (it has been 4 loooong years, in my defense, lol).
This photo makes the colors more muted than they were, but you can see how they were not far off in tone from each other, since it muted both colors equally. BTW - the p.o. put in the kitchen and fixtures. We only painted the walls from a pale dead beige-peach into the more vibrant purple which was picked up by the whitewash cabs and floor and looked good next to the dated sea foam counters. In this pic, you can't see the purple tone in the floor tiles. We also replaced the old fridge, but otherwise, this was not our baby. I did not like the decor of it too much, but it had twice the space of our wretched, uglier, current kitchen and the layout worked really well. Our new kitchen will have the space, layout AND be pretty (we hope). I can't wait to unpack half my kitchen stuff that is still in boxes because of such tight quarters, when our new kitchen is in ;)
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thanks kitchenkrazed, my island is a "baby", it is 48" x 24" (just 2 cabinets put next to each other),with a bit of an overhang, so maybe 52" x 28"?