There's a lot of popular interest in color psychology and in the case of the humble yard sign, it's one of the few decisions that a campaign needs to make. It's important, however, to put colors' influence in context. There are a number of other factors to consider when choosing the best color for a campaign sign, including choosing colors that are easy to see and contrast with one another and that are in line with the rest of the candidate's brand and other materials or not.
Also, the campaign's budget and other campaigns themselves can limit your color choices before you even consider whether your color choice conveys the meaning that you'd hope for. In the case of budget, if your campaign is choosing to go one-color, your options are limited. You might like the idea of using purple because it conveys wisdom, but especially depending upon the shade, it might not work with a standard background color.
I recommend avoiding patriotic colors on campaign campaign signs especially if there are a lot of other campaigns, such as in a local election year, using red and blue. It's hard enough to draw the attention of voters to your sign without printing a sign that looks substantially like any number of other candidates that are running for elected offices in your district.
If after those considerations you are debating between a few colors, this post I wrote is an overview of what colors on yard signs convey psychologically. To summarize:
• Black conveys power and authority. It's also easy to see from the road so it's a good choice in any district.
• White is a fine background color, but it's associations with concepts like purity and sterility aren't especially meaningful to voters.
• Red means power, think "power tie," and also has strong connections with love. As such, if you aren't going to drown in a sea of other red, white, and blue signs, red is a good choice.
• Blue is a cooling and calming color. Hopefully, voters aren't that angry with you that you need this effect.
• Green is a symbol of the natural world. Candidates who want to connect themselves with environmental issues are smart to choose this color.
• Yellow is a bright and cheery color that when paired with a dark and contrasting color, works well in most districts.
• Purple conveys wisdom but also royalty. Grassroots campaigns beware.
• Brown, like green, is an earthen color that makes people think about the environment.
• Orange conveys excitement, warmth, and outside of psychology, it's an attention getting color that works well on many signs.
Finally, it's important to consider that there are different views on how much impact color has on people. Certain people are more influenced by color than others and depending upon someone's mood and their personality their emotional response to color will be different than someone with a different personality or who's in a different mood.
Ben Donahower writes about campaigns signs from a political operative’s perspective at Campaign Trail Yard Signs. Some campaigns get advice about signs from a printer, but Ben makes campaign yard sign recommendations to candidates rooted in political principles. You can connect with Ben on most social networks including LinkedIn and @iapprovethismsg on Twitter.