Lyric discussion by xmod2 

I think it's about him losing someone close to him (lover) and the despair and depression that follows.

A line of cars painted black. Funeral Procession.

With flowers and my love both never to come back...

the rest of the song talks about he it outwardly projecting his darkness.

I think you've got it right xmod2.
Additionally, I think the red door is love...and it's because of his love he is suffering.

I look inside myself and see my heart is black I see my red door, I must have it painted black

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