You may advertise on purple.com. Why you would do so is somewhat different than advertising on other sites, so please do read these guidelines.
The cost is 50 per day for the first two words. For longer ads, the price is 50+50*(w-2)^2 per day. I.e., 100 for three words, 300 for four words, etc. For images, the price is 0.10 per pixel per day. So an 88x31 tag (2728 pixels) would cost 272.80 per day. We are interested in the page loading quickly.
You may only advertise on the root page. That page receives about 15,000 views per day (of which some are surely robots and other automated entities). Extensive anecdotal evidence suggests that the root page is viewed by people testing their browsers or simply playing with common English words. Changes to that page tend to elicit comments and blog posts, so at least some people visit regularly and notice changes.
Much of that traffic depends on the page being essentially blank purple and loading very fast. I therefore require advertising to respect those constraints. As such, you are limited to three words (and to about 20 characters, at our discretion).
Since there is no substantive content on the root page, I can't help you target your advertisement to your audience. I suspect the potential interest is precisely the fact that you would be paying to reach an unknown audience, for example to publicise a new product launch.
In addition, I require that neither the words of your ad nor the initial page to which they jump should be obviously illegal in France, the United Kingdom, or the United States. They should also not be likely to offend too seriously in those countries. (So please don't ask us to link to a cartoon of Mohammad or something that will cause us to be hassled, even if I take no personal position on your content.)
If you want to link through to something questionable or NSFW (not safe for work), please link through an intermediate page which provides clear and adequate warning of what's about to happen (in English) and does not redirect for at least five seconds. That should be enough to let people who don't want to continue on to abort.
The pricing may vary with time and is designed to make sponsored links the exception and not the rule. (That simplifies my life for now and increases the effect of your advertisement.) It is currently 50 euros or 50 pound sterling or 65 U.S. dollars per day. If you are paying in Europe, I would prefer payment by bank (SEPA) transfer if possible. Otherwise, we can use paypal.
I am not currently planning on automating this service (beyond using cron to swap in your ad and pull it out again), even though my fingers twitch to write a simple word auction system. But I don't think that's worth my time. If you are interested in advertising, please email me, username "jeff". Please don't spam me. Please use a clear subject. Please contact me in advance of when you want your ad to run so that we have time to handle payment and set up the page.
If you send me mail that does not appear to be constructive or serious, I may simply ignore it (and flag it as spam for my spam filter to handle next time).
I am open to constructive feedback on the advertising program. My purpose is to cover my (miniscule) costs of running the site. If I buy myself some good chocolate out of it, that's a nice plus.