http://www.ehow.com/how_4525032_use-color-wheel-decorating.html
A color wheel can be intimidating because its circle of hues throws a vast amount of color options into a decorating plan that may already seem weighed down with too many choices. However, a color wheel reveals relationships among colors. Once you understand those relationships, you’ll build color confidence and understand how well-chosen colors can change the feel of a room or add interest to a decorating plan just by pairing them with a coordinating accent color.

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Difficulty: Moderate
Instructions

1
Begin by understanding the primary colors, which are red, yellow and blue. All hues come from those three colors, and working with them in their pure form can be difficult, but some planning can help. For example, yellow works well with texture and typically looks better on a textured wall than on furniture. Yellow is affected by the colors with which it is matched, and it partners well with green and deep blue.

2
Consider that while red can be dramatic, it also can overwhelm a decorating scheme. However, analogous colors can be used to incorporate understated reds into a room that are easier to live with than primary red. Analogous colors are side by side on a color wheel and include red-violet, red and red-orange.

3
Use blue to change the feel of a room. The look of blues is affected by the colors with which they are combined. Blues look warmer when paired with red. Blue and red are complementary colors, which are directly opposite each other on the color wheel. Warm color combinations can make a large room feel cozier.

4
Create an analogous color scheme that makes a small room feel larger. For example, blues appear cooler when combined with greens. Since cool colors retreat, they can make a small room feel larger.

5
Add interest to a decorating plan by working with three fabrics in analogous colors and accenting the trio with one of the opposite, complementary hues on the color wheel. Consider this option for sofas combined with pillows and throws.

6
Choose a triad color scheme to add an appealing balance of hues to a decorating plan. Three colors that are equidistant on a color wheel create a triad and include green, violet and orange. This trio of colors may seem to clash, but working with them in darker hues makes them more appealing. Consider forest green, plum and burnt-sienna.

If you paint the arch a color, it will appear smaller. I'd paint it same as the ceiling in both rooms.

Thanks for the help all! This is OP. We painted it last night and decided to go with the white (same color as the ceiling and trim) and I adore it. The arch is really pretty and I think it kind of highlights the arch instead of having it blend in with one room or another. I guess there is no "right" way to do this though so all the comments were very helpful. Appreciate it!

We have this in our home. Archway to dining room is Tiffany blue just like our living room. Dining room is lime green below the chair rail and barely lime above it.
It looks great.

Thank you for the advice on arches. Seriously, I have them throughout my older house and I was never sure how to paint them!

Anonymous wrote:OP, I have this situation, with a trim-less plaster arch (shaped like a true arch, curved over the top, with no inflection point between the top and the sides) between my hallway and DR. I painted it the hallway color because that's the direction from which you usually see it, and it frames the DR nicely. In your case, I would go with the yellow as you probably spend more time in the LR looking toward the DR than vice versa, and the yellow is also more neutral than blue.

This is the same advice I saw on an HGTV makeover show!

OP, I have this situation, with a trim-less plaster arch (shaped like a true arch, curved over the top, with no inflection point between the top and the sides) between my hallway and DR. I painted it the hallway color because that's the direction from which you usually see it, and it frames the DR nicely. In your case, I would go with the yellow as you probably spend more time in the LR looking toward the DR than vice versa, and the yellow is also more neutral than blue.

We had this situation, and we did as pp's suggested. The underside of the arch was white (which matched the trim and the ceiling). We also had wooden trim along the edge of the arch on each side. You can add it pretty easily if it fits with the architecture of your house. Our house was a victorian row house, and we had chair rails and crown molding in both rooms, and the trim around the arch coordinated with that.

I'd paint each side to match the rest of the room, and then paint the inside of the arch to match the trim.

I would paint each side of the arch the same color as the room, with ceiling color on the arch's underside.

We are painting our living room and dining room, which are connected by a larged arched opening. The arch does not have trim - it's just a continuation of the plaster walls, if that makes sense. The living room is yellow, the dining room is blue, the trim is white. Do I paint it white? Or blue? Or yellow? Thank you!