Red, green, and blue
Red, green, and blue are the colours of Wikimedia. The Wikimedia colours keep Wikimedia projects together by linking many brands in a consistent look. See Brand#Using the Wikimedia colors for the specific color values. Even if not all Wikimedia logos are red, green and blue, such as Wikipedia, Wikinews, Wikitionary and etc., most of the logos are still based on these colours.
Originally designed by Neoluxx (File:Ncwikimedia.png on 15 October 2003).
Precursor to the original (File talk:Ncwiki.png), as part of International logo contest/Logos 1-25
Wikimedia Indonesia Community Logos
tool labs (old)
Derived from Community Logo
Unused sister project logo proposals
Wikimedia Strategic Planning
From Logo for Wikimedia Strategic Planning wiki.
From Wikimedia Incubator/logo.
For brevity, not all variations are included in this gallery. You can see them on Wikivoyage/Logo 2013/Submissions, and previously Wikivoyage/Logo/Round_1 and Wikivoyage/Logo/Round_2
From Wikibooks/Logo/Archive 1.
From Wikidata/Logo voting
More sister project alternatives
Committees, Chapters, Usergroups, WikiProjects, Raccoon Lodges
Why don't we use these colors everywhere?
It was discussed during the 2006 Wiktionary logo vote, and was determined then, that having all the logos use the same color schemes, would be confusing; see example at w:de:Benutzer:Julian~dewiki#Test on logos. See also, these discussions (Discussion on the logo votes#The Color Complication and Talk:Wiktionary/logo/archive-vote-4#Changing colors), in particular this comment: "the only thing we can do is to ensure that newly chosen logos harmonize [with] the others but don't look all the same". See also Logo#Logo guidelines: "It must be substantially different from the other project logos but still harmonize with the others."