Telepathy questions

[Farwalker]

I'm trying to figure out how best to use telepathy, or how it works in general. Couple questions, which might be easy:

1) What blocks telepathy? I did some testing in a town with a telepathy: all randart on, and couldn't see people on the other side of buildings. Then I went and hunted down an adventurer party (er... I mean "I was ambushed"), and I couldn't see them in the unexplored black area. I could see them on the other side of trees once the areas they were in were explored. However, in other cases I've seen critters in the unexplored dark with telepathy, so I'm not sure what's going on.

2) What's the significance on the status page of a telepathy in green (for example dragon) or in yellow? If I wear two dragon telepathy items it goes yellow, but is that indicating the range is bigger or something? It's really not that clear (would be nice to see what the range actually is somewhere).

Thanks!