Another idea, based on this presentation about combining music and colour synaesthesia and Moodagent:

The idea is to assign colors to songs. The color you pick can be based on intuition, a memory of the videoclip, a color in the song title, etc. The typical features of the application could be the following

1. Browse the musicotheque (Tunify music library and assign a color to each song you play
2. The songs in a playlist are annotated with the color you picked.
3. Users can annotate each color with tags (to express what they feel when thinking of it)
4. Playlists can be generated by selecting one color or a range of colors.
   
5. The song - color combination could be stored in two ways (maybe both ways can be used?):

• Locally: each user has a personal database for storing his song/color information. The advantage of this is that the used colors and the song genre will be more consistent and the user has no songs from others interfering with his music library.
  
• Globally: This means that when the user generates a playlist by color (red, for example), there might be songs in it which he personally would tag with another color, such as green. This means that he might not get the music he expected, as people in different areas of the world experience different feelings with certain colors. Nonetheless, it might be an interesting way to discover new music and to see what other people assign to colors. Also, this way statistics could be generated which could show the most popular songs for a certain color, or a diagram of all the colors ever given to a certain song and their impact.
  

I think the information should definitely be stored locally, to give the user full control over his music. The possibility to access color/music information on a global scale is optional, but could be very interesting.