Last week, I started talking about things I consider when creating or evaluating a ballroom costume, and promised that I continue with talking about evaluating colors this week. I find color is one of the first things I consider when coming up with a potential dress design. In my head, certain colors only work with certain designs; something that would look elegant in royal blue is going to give a different impression in hot pink. That is probably a peculiarity of my thought processes, but I would still say that the color of a dress deserves careful consideration. At the very least, fabric cost is almost always independent of color, so I consider color choice as the cheapest way to add visual appeal to a dress.
I stated in my previous post that I felt one of the tasks of a competition costume is to attract attention. The color of a dress can do this by contrasting with the surrounding colors. To know what colors contrast, consider a color wheel. A color wheel displays relationships between primary, secondary, and tertiary colors. The three primary colors are red, yellow, and blue. The secondary colors are produced by mixing two primaries and are green, orange, and purple. The tertiary colors are made by mixing secondaries and primaries, though exactly what shade you get depends on the proportions mixed; yellow-green, orange-red, and so on fall in this group.
One important aspect of color theory for this discussion is that colors on opposite sides of the color wheel are complements, and complements in close proximity make each other appear brighter and more vivid. They stand out more to the human eye. Pairing a blue with an orange will get you the most visual impact from each. The same is true for yellow and purple, and red (or pink) and green. On the other hand, colors near to each other on the color wheel will tend to visually blend; yellow and orange near each other will merge when seen from a distance.
You can make your dress stands out more, then, by picking colors that contrast a lot. Pink and green is a common pairing for this reason. You can also contrast against a neutral; black and white is the maximal contrast there, but white and fuchsia or burgundy, or black and hot pink, are also stand-out combinations.
I would also recommend considering your surroundings on the dance floor when evaluating contrast. There is about a 98% chance that your partner is wearing black. The dance floor is normally a tan color. Gym lights tend to be rather yellow in color, while some dance schools have disco lighting that throws off all colors. If you have the chance to do so at some competition, I suggest closing your eyes, looking above the floor, then opening your eyes and seeing which color attracts your attention first. In doing this at several competitions, I've found the yellow is a pretty consistent first glance for me, followed by hot pink and lime green (watch any of DSI's videos of first rounds at Blackpool--lime green dresses are so bright the camera has trouble filtering them). The neon colors also tend to retain their brightness under almost all lighting conditions. Red, while it seems to be a color that would attract a lot of attention, tends to dull under adverse lighting conditions and doesn't always stand out that much.
The first practical conclusion that follows from these considerations is that black is a dangerous color. Consider this showcase. Katusha is wearing a bright yellow dress that contrasts strongly with her black sleeves. However, when she comes into frame, to me it looks like her arms disappear since they blend with Arunas's. I find the look distracting. Rooms can also have dark walls. I was at the Dutch Open watching a friend over a year ago, and struggled to find her on the crowded dance floor during the amateur latin events. She was wearing a black dress, on a floor with more than a dozen other people also in black, against dark colored walls. The black dress wasn't helping her stand out.
However, I do think it makes sense to use the fact that your partner is wearing black as a source of contrast. White, pink, and lime green dresses will always show up against your partner's attire; I picked jade for my last dress in part because I like how it contrasts with the black of my partner's tailsuit.
I've listed mostly bright colors as being good choices here, and I offer them as being the most likely to ensure that you and your dress get seen. Other colors can also work. Pastels and jewel tones are common, particularly among standard dancers; it's hard to miss a pale blue dress when there are several square meters of fabric swirling past you, even if the color isn't very bright. Latin dancers, with smaller dresses, have less area to play with. If you want a color that isn't so bright, I would suggest paying careful attention to other sources of contrast to ensure your dress still packs a punch.