User blog comment:DaNASCAT/Technical Update: August 15, 2016/@comment-1142365-20160816153215/@comment-5275700-20160816161214

The grey is the best option to both compliment the new Fandom color scheme and to be a neutral tone that will work with most wikis.

Consistency is important for branding, especially since the Fandom brand is so new. It's impossible to find a color that would work with every wiki theme due to the wide array of customization.