Solved by reading docs -> https://nav.uninett.no/netmap. 

The colors are based on rrd files generated by cricket. But, my device is slow to anwser snmp get-bulk requests. So, to solve the problem, I've to increase the timeout value of cricket's Default configuration to 15.0:

/usr/local/nav/etc/cricket-config/Defaults

snmp-timeout        =    15.0




2013/3/21 Bruno Galindro da Costa <bruno.galindro@gmail.com>
Hi!

    Which is the premisse for netmap colorize the link between two devices? What is the meaning of grey's link color?
    I'm wondering this because from yesterday to today, the links between my devices changed from green to grey...

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Att.
Bruno Galindro da Costa



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Att.
Bruno Galindro da Costa