I created a service script which starts downloading a song in a background process as soon as its added to the playlist. So during initial adding, the song doesn't exist on disk and therefore (I think) shows up grayed out in the playlist. After successfully downloading the song it can be played normally but is still showing up gray. How can I make amarok recognize that the song is available. Or is something wrong with the way I add the song?

The value of the playableUrl field looks like this:
file:///home/user/.kde4/share/apps/amarok/scripts/myservice/audio_cache/randomName.mp3
and when I look at the added track's details, the filename shows up correctly.

When I remove the track from the playlist and add it a second time with an existing mp3 file, it still shows up grayed out. What am I missing?

Is the track only downloaded to disk or is it actually added to the collection? And I presume by playlist you mean a saved playlist on disk or the play queue?

Please also state your Amarok version and Plasma version when you ask a question

Hi

* The Track is only downloaded to disk into a temporary directory that doesn't belong to my collection folders.
* By "adding to the playlist" I mean dragging the song from the Media List of the Service to the main Amarok play queue.

When opening the "Track Details" window, in the notes field it claims that the file doesn't exist and therefore can't be played but when I double click the Track it plays normally.

I'm using OpenSUSE Leap 42.3 with Amarok Version 2.8.0 and Plasma 5.8.7

I suggest you upgrade your Amarok version to 2.9, there have been a lot of improvements, and Amarok 2.8 is about 4 years old now. Maybe that alone will already help.
I am not sure a temporary directory is a good target, can you see the file in the file manager from within Amarok?