Interesting question. I used to have something similar to synaesthesia when growing up, in that certain sounds would cause visual hallucinations. But I never saw colours or shapes, it was more like I was on drugs and things would move that weren't supposed to, like paintings, walls, statues etc. Not a nice thing for a kid to experience. Unfortunately, by the time I'd gotten into Moz, that little quirk had disappeared.
If I were to visualise Morrissey's music though it would have a sense of urgency about it. The Smiths always conjured up either people sprinting wildly around the countryside, grey days by the seaside or scenes far removed from my life, like battlefields. Sometimes I'd visualise a Pied Piper like figure being followed into water ... but that's just me imagining Morrissey metaphorically drowning his fans. However, if the Smiths were a colour, they'd be blue.
Alone, Morrissey has always been more about cities and people. When I hear a Morrissey song I see a person taking in the sights and sounds of the city, whilst alternately avoiding, or embracing it's citizens. Saying Morrissey sounds grey or silver, seems like I'm saying he sounds boring, but that colour sums up the city for me (metallic, cold, but beautiful to observe). It also sums up his world; nothing is black or white, life is endless shades of grey.
You asked me what his songs looked like, and I had this imagery in my head. *sulks*
But I'm still sulking a bit... 
