Yes the brown dissolves in cold water too (not completely nor nearly as quickly as #4 white obviously). It most usually comes in rock/hard clump form, the colour varying from greyish-brown to light brown. Purity levels vary in the North from 5% (only got that once) to 20-25% maaayyybee but average at 10-15%, so basically shite. The solution is very clear yes, completely transparent and nearly colourless. Btw this refers to northern Italy (specifically Milan, but it isn't the H capital of the north, don't know what else is available elsewhere in the North but all Milan dealers stock very similar shit).
The tar can only be found in southern Italy, where both the brown and the tar are much higher purity (30-40 % ) and the purity is far more consistent (the presence of the mafia is more felt and it's a tighter-knit community with fewer dealers than a big city, so reputation is very important for both reasons). Tar obviously requires heat, the brown is also different, more on the grey side than actual brown.
I'm inclined to agree with blondin's hypothesis, both due to the presence of tar on the market and because Italian - South American mafia links are very strong, so it'd make perfect sense that we get our H from there, and it would also explain the less refined look of the smack in general ("China White" is nothing more than an urban myth).
What doesn't make sense to me is why the UK doesn't get their shit from South America too. Central Europe/Eastern Europe I can understand because once it's in Turkey well the job's practically done, but it doesn't make sense logistically that you guys get it from central Asia...
[Edit] blondin re-read your post, and the tar and the brown are definitely totally different (besides appearance which can be pinned down to different cuts) - the actual stone + taste when IV'd are very different - tar in particular. A friend of mine in the South who has been shooting for 20+ years also confirms this.