what are the different colors in Sharing Hosts (Search page)

The colors depend on the type of network the file
is on. Here private networks are in yellow. I remember
the ip-address 0.0.0.0 was in red.

All invalid IP's are in red. You will never be able to directly connect to them (only with push). Private IP's are in yellow you can directly connect if they are in your local network if not only push will work.
The chance that a push file transfer will work is very low compared to a direct connection. It will only work if the complete way of the connection hierarchy can be followed back. If one host on the way droped the connection to the next host a push request will fail.