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tl;dr yellow means bad, orange means good but might get bad, white is fine (in a pretty generalized summary, that’s how I understand it at least).

https://developer.valvesoftware.com/wiki/TF2_Network_Graph
Quoting:
The indicator will turn yellow if the server’s framerate (actual framerate on the remote machine) drops below this interval [talking about the interval shown by “lerp” in the net_graph] . This can be used to figure out why all of the objects in the world are no longer moving smoothly. In addition, the indicator will turn orange if the user or server has tuned the ConVars such that the interpolation amount is less than 2 / updaterate. This indicates that if there is any packet loss (or possibly choke if the choke is occurring for long periods of time due to large packets being sent over a low bandwidth rate setting) that the player will likely see sluggishness in the game.

Now that we are at it, I hope you don’t mind if I ask a few more questions.
Quoted from octochris

You can receive more updates than 66, it’s just that that is the maximum tickrate.

Tickrate is (technically iirc) 66.666…, so what would that mean? that in optimal conditions the updates would vary between 66 and 67 per second? If that’s correct, is 67 the maximum number of updates, or is it some other number (or maybe no limit?)?
Quoted from octochris

It’s not really a client side “lag”, it’s more of an “averaging” of the data (but yes, this does have a comparable effect).

I understand the concept of interpolation, but the value one uses in cl_interp is used as the time the client has to interpolate, right? (That’s why I said a “client side lag”, you delay the sending of information to the server by ‘cl_interp’ seconds).

I hope I don’t end up being annoying, I just want to really understand all this so please correct anything I say if it’s wrong, even the tiniest detail.

Quoted from octochris

You’ve got it the wrong way around. Interpolation happens to entity data being recieved, not entity data being sent.

Yeah, my bad, edited that last part in a rush because I had to go and wrote something that didn’t make much sense. I know the data you receive is the one being interpolated but… damn I got it mixed up now lol.
Why is it that if you set the maximum cl_interp possible (I believe it’s 0.5), just for testing purposes obviously, projectiles have a significant delay between the moment you shoot, and the one it leaves your weapon (at least that’s what you see client-side)? That’s why I said you delay your client ‘cl_interp’ seconds.

Quoted from octochris

[…]

Because of lag compensation.

More the lack of it for projectiles, so when you increase your interp its like playing with a higher ping when you fire and see them since it means you are playing the game further in the past so to speak.

Oh and the reason for the recurring number is because the game sends a packets at a maximum rate of 1 packet every 15 ms which works out to the 66.666. Hz

FireStorm

Citations
http://wiki.teamfortress.com/wiki/Lag_compensation

Quoted from octochris

[…]

That’s not strictly true, the server ticks at that rate, but there is no engine design guarantee that says the same about packets.

I didn’t think I would get away with it but yea, server ticks is more accurate the packet rate does indeed vary.