One of Wikipedia's strengths is that it has a large number of pages wikilinked together for easy cross-reference. The fact that they turn red or blue depending on the page's existence helps users avoid visiting a nonexistent page if they are not interested in creating a new article. However, this does not work with links to other wikis.
A fundamental issue that has always existed in Wikipedia politics is the notability question. A standard answer to those who question deletionist policies is "Go start your own wiki and put your non-notable content there." This is not a bad idea, but all links to Wikipedia articles will need to be set up as external links; and these outbound links will not turn red or blue depending on the page's existence on Wikipedia; they will merely be light blue at all times. Moreover, a page might exist on the local wiki and then later an article on the same subject, with the same title, will appear on Wikipedia. It is desirable that capability be established for the colors of the wikilinks to reflect a page's presence both (1) on the local wiki and (2) on Wikipedia.
It should be possible to do so without putting as great a burden on Wikipedia servers as one might expect. The reason is that the local wiki need only maintain a current list of pages existing on Wikipedia and then compare any inquiries to that list, without having to contact Wikipedia every time a page on the local wiki is accessed. Obtaining a snapshot of the list of pages existing on Wikipedia at a specific point in time is a one-time event, and then logs of page additions and deletions can be obtained periodically to keep the list in sync. The rest of the mechanism should be a matter of checking the page name in question against (1) the list of pages on the local wiki and (2) the synced list of pages on Wikipedia; and then turning the wikilink the appropriate color.
This type of enhancement could also be useful for links among the various Wikimedia projects. It is desirable that a link from Wiktionary to Wikipedia, for instance, turn red if that page is subsequently deleted from Wikipedia. That will alert a reader who has no intention of creating such an article not to click on that link, since the link is essentially broken, as far as he is concerned.
The proposal is also summed up at http://wikunamatata.com/wikidev/index.php5?title=Red-green-blue_project