You must have previously installed a mod which changed the shaders package for Oblivion.
There are some depth of field mods that do it, and a few others. Chances are it is an omod file.
If you can, you can deactivate that omod, then try reinstalling the patch again, then look for an updated version of the mod, or just try reinstalling the omod afterward.
If you can't do that, then you'll have to look on TESNexus for a file that contains the original Shader packages, and copy them into the shaders directory of Oblivion/Data.
It's a common problem.
BTW, grab the OBSE 18beta6 package. It's nearly finished and a bunch of mods are dependent on it now.
edit: one more thing. For a laugh, look closely at the ground. The purple is actually a bunch of miniature purple versions of the map of the Imperial City repeating over and over.