Selected answers from the Dumb SEO Questions G+ community.
Walter Mrowczynski III: Default is red, blue, and purple, but as a designer you set those colors based upon what other colors your site is using. If you don`t know what colors are harmonious with the rest of the site, use something like Palletton. http://paletton.com
David Kutcher: Yes.https://www.nngroup.com/.../guidelines-for-visualizing.../https://www.nngroup.com/articles/clickable-elements/Old, but holds true still.