Well, the short answer is that you can't, not with AutoCAD alone. However, there are some add-ons that might help. For example, if you have the AutoCAD Express tools, you can use the "Super Hatch" command to use true colour image tiles as your hatch. See http://www.cadtutor.net/acad/acad2k/map/map.html#Super for details. There is also the M-Color add-on http://www.motivesys.com/.
I must admit that with every new version of AutoCAD, the expectation is that the 255 colour limit will be broken but so far it just hasn't happened.