In my oppinion, the advantages of a coloured song is that you can highlight ONE proporty of a song:
If I had a track marked tempo=fast=255,125,0 (orange); mood=energetic=255,0,0 (red); occasion=workout=255,0,255 (magenta)
would MB show all 3 colours as a gradient, pick only the first colour or do a blend of all the colours that hopefully wont match a colour that is already tagged for something else?
With too many colours in one song or with a blending of colour :
- you could get confused with the meaning of many colours
- you would not see directly which colour the song has, so you wouldn't have the effect of seeing directly why the song is marked.
-With many different colours in one song you would have the same problems as with the written tags: Too much information for a fast recognization of the "special property" (like slow BPM, no artwork) you're looking for
But I realize that it would be great to have different "colour-rules" at the same time but that there would be interferences for songs which would apply for 2+ colourrules.
Possible Solutions:
- Have a colour priority list, so that a song, even if it applies to two colour rules would one show one colour rule dependet on the priority list
- colour the tags if you had more than one colour rule applying to the song (A song which is slow would have the BPM tag-field coloured blue; if the genre is IDM, than IDM would be coloured red)
- To make some Tags BOLT instead of two colours, so that a song which is slow would be completly blue; and if the genre is IDM, than IDM would be written in BOLT)
Also I would like to say that i really like Gregs mashup with the Drop-down Menu
Potential usefulness aside, this has the possibility of making MB look very ugly! 
if youd had custom colours you could make them even look nice in the skin. I get you're point, but even if it would be ugly, for me, usefullness is more important than a good-looking skin.