Help:Colon trick
The colon trick is a method of creating a link to a category, image or interwiki link. It does this without adding the page to the category, displaying the image or adding the interwiki link to the interlanguage links. It also allows for linking to pages with titles beginning with a recognized external link prefix such as http://.
By adding a colon just inside the opening double square brackets of the link, it changes the way MediaWiki software normally works. It will appear as a regular wikilink.
Examples
change[[Category:Help]]would list this page in Category Help but[[:Category:Help]]will produce a link to Category:Help[[File:Wikipedia-logo.png]]would display an image but[[:File:Wikipedia-logo.png]]will produce a link to File:Wikipedia-logo.png[[pt:Internet]]will create an interlanguage link but[[:pt:Internet]]will produce an interwiki link to pt:Internet (in the Portuguese Wikipedia)[[//Hus]]will produce an (invalid) external link wrapped in brackets (i.e. [[1]]), but[[://Hus]]will produce a link to //Hus.
With templates
changeA colon can also be used to transclude a page in mainspace, rather than template namespace. For example, {{The Beatles}} will transclude Template:The Beatles, but {{:The Beatles}} will transclude the mainspace article The Beatles. This is useful when the mainspace article has been specially formatted to be transcluded in other articles, by using <noinclude> or <includeonly> tags to indicate the portion that should be included when the article is transcluded. Welcome vs Welcome