It is hard to decide how to paint a common wall shared between two rooms. If your goal is to visually distinguish between the two rooms, it won't do to stick a vertical piece of tape in the middle of the wall and paint different colors on either side. The trick is striking a balance between breaking up the rooms and maintaining an aesthetically pleasing flow. Your first step is to look for examples of open space concepts in magazines or in others' homes you like. Next you must envision what the two rooms will look like from all possible vantage points for each possible color choice to determine what matches and what clashes.
Paint the shared wall one solid color that matches the color schemes of the three remaining walls of both rooms. The two rooms will be distinguishable since each will have its own paint color on three of the four walls. For example, you could paint one of the room's three walls straw yellow, the other room's three walls a sky blue and the common wall a royal blue as all three colors match. Another option is to paint the common wall a neutral color, such as white, beige or cream, and to paint the rest of the rooms's walls complimentary colors--colors opposite one another on the color wheel.
Break up the common wall where the two rooms meet with a vertical structure and paint on either side. Use a vertical molding, such as a white pilaster, or a piece of furniture such as a floor-to-ceiling wooden shelving unit. Paint the common wall on either side the same color or each half the same color as the room it is part of.
Paint the entire common wall a solid color, either neutral or a matching color to the two rooms, but continue this color into both of the two rooms. Paint the bottom two thirds of the wall perpendicular to the common wall the same color as the common wall. Do the same for both rooms. Paint the top third of the perpendicular wall the same color as the other two walls of that room. Use a level to ensure you apply the painter's tape in a straight, horizontal line. For example, one room could have two solid cranberry red walls and one wall with the top third cranberry and the bottom two thirds burnt orange; the second room could have two sage green walls and one wall with the top third sage green and the bottom two thirds burnt orange; the common wall would be completely burnt orange.