A yellow light means your client can reach other clients but other ones can not reach your client. Your can still download with BitComet in this case, but your speed can not be as high as in green light status.
The most possible reason of a yellow light status is that your pc is in an intranet. So you have to configure a NAT (Network Address Translation) for BitComet listening port on your gateway. Another reason is the firewall used by your system or your ISPs system blocked this port.
These guides provide step-by-step guides on how to setup port forwarding. http://www.portforward.com/english/routers/port_forwarding/routerindex.htm
