If you're looking to apply the same effect to a series of images, have you tried the "Batch Queue Manager"? This will let you select the effects you want to apply, and from the "Queue" menu you can "Save workflow".

If you're looking to apply different effects to some images, and other effects to others, you could save them as two (or more) separate queues.

If you're looking to apply different effects to each image, I'm not sure that DigiKam does that. Personally, I like the way I can avoid this in DigKam - adjusting each image as I go. If I wanted to do that, I would use Darktable, which saves a .xmp file with each - though Darktable can be heavier going than DigiKam for image management, so I tend to review images and do some editing in the DigiKam Editor (basically an in-DigiKam GUI for Showfoto), but for more specific types of editing I'll use Darktable (which can be opened from DigiKam with a right click and "Open with" on each image).

Other editors and viewers are also supported by the right click from DigiKam - for example Rawtherapee and GIMP (though GIMP is more of a pain with RAW files).

It might be that others here will know a different trick, but since I've found that DigiKam is good for image management and Darktable for more specific editing, I use a combination of the two in, if you'll excuse the hi-fi metaphor, a separate-turntable-amplifier-and-speakers kind of approach.