The very basic requirement here is that the Link should be colored Red when either port associated with the Link is reporting "Down".  Down will be determined by the value of the SNMP object: ifOperStatus.  More advanced state setting capabilities should be considered, i.e. LinkStatusProvider in GraphML Topology, but this is the minimum requirement.  RFC provided below.

 
McCloghrie & Kastenholz Standards Track [Page 31]

RFC 2863 The Interfaces Group MIB June 2000ifOperStatus OBJECT-TYPE
SYNTAX INTEGER {
up(1), – ready to pass packets
down(2),
testing(3), – in some test mode
unknown(4), – status can not be determined
– for some reason.
dormant(5),
notPresent(6), – some component is missing
lowerLayerDown(7) – down due to state of
– lower-layer interface(s)
}
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The current operational state of the interface. The
testing(3) state indicates that no operational packets can
be passed. If ifAdminStatus is down(2) then ifOperStatus
should be down(2). If ifAdminStatus is changed to up(1)
then ifOperStatus should change to up(1) if the interface is
ready to transmit and receive network traffic; it should
change to dormant(5) if the interface is waiting for
external actions (such as a serial line waiting for an
incoming connection); it should remain in the down(2) state
if and only if there is a fault that prevents it from going
to the up(1) state; it should remain in the notPresent(6)
state if the interface has missing (typically, hardware)
components."
::= { ifEntry 8 }