There is no way to tell what to connect to. It depends on how they wired it. I think it is safe to assume you should connect to the single black since it is in that box by itself like that. And yes, you would connect to the white and green with the other two wires. The instructions are not sparse, they show what you need to connect to. They do not know or care how your house is wired, nor should they. I will say, that box for the porch light is NOT appropriate. That is a collar box meant to mount onto an existing flush box. The wood of the logs is exposed to the back side of the box.
I also see a hole in the back of that collar box. Is this new light meant to be inside or out? If outside you'll need to replace that existing Wiremold collar box with a regular box, then run a cable from the back of the box into the back of a flush box outside.
Conventional Switches
It sounds as if most of the switches you have been replacing are single pole switches. That is, they make (on) or break off) a single connection from one location.
Simple switches use a black (hot) and another wire, usually black, red or blue (switched hot) to make and break the circuit. Basic switches do not need a neutral wire (white), so the neutral in a switch box is sometimes capped.
Smart Switches
Smart switches, like the Insteon, often also require a neutral (white) wire. The ground wires should be present and connected on all modern switch connections.
Conventional 3-way Switches
When a light is controlled from two locations, you need a switch called a 3-way. A basic 3-way switch has a black wire or connector, called a common, and two traveler wires, usually black, red or blue. The two travelers may be the same color or different colors, but the switch itself will usually be marked. It also should have a ground.
The common wire on one of the 3-ways is connected to the incoming hot line. The common on the other switch is connected to the fixture or outlet being controlled. The two travelers are connected to the traveler terminals on the opposite switch.
This setup allows either switch to change the state of the fixture by changing which traveler is connected. When both switches are on the same traveler, the light is on. When the switches are on different travelers, the light is off. A flick of either switch can make or break the connection.
When you want a smart switch, the 3-ways often need to add a neutral (white). This is always connected to the neutral line of the circuit (other white wires) and not to a hot or a switched hot wire.
WiFi Switches
If you want to control a fixture from two locations with an Insteon type switch, you need 3-way setup. However, these types of switches do not need two travelers, just a single connecting wire. They communicate the change of state (on or off) wirelessly between the switches, and the hub if it uses one.
The wiring directions for a 3-way connection can be found here, at the Insteon site. The following is a picture of the 3-way setup.
You can see that at the switch where the hot line comes in, the line hot, switch hot, and one of the traveler line (in this illustration black) are all connected. The other traveler (red in the illustration) is not used so it is capped.
In the second box that holds the cable to the fixture, the active traveler from the first box is connected to the black to the second switch, and the red from the second switch is connected to the fixture. The second, unused traveler is also capped.
If your two travelers are both red, you need to figure out which one is the connected one and which one is the unused one. Sometimes the wires have a marking, such as a stripe to help distinguish. If not a simple trial and error approach will work. If you pick the wrong one for the second switch, it simply will not work. you can then swap the traveler wires in the second box and you should be fine. Be sure to cap the unused wires.
Best Answer
It looks like a standard "power on the end, lamp in the middle" 3-way arrangement. However (for whatever reason) they chose to make the travelers black and white on both segments.
Travelers can be quite a dog's breakfast, I've seen 3-way circuits where the travelers change colors on every segment. Choosing the same colors thru makes some sense -- however white should not be one of the chosen colors. though the far, far better way is to pick a colored tape color such as yellow, and mark both travelers yellow (they are interchangeable, they can be the same color).
That's because white is special in Code rules. Colored wires (hots) cannot be remarked to be neutral. But white wires can be remarked to be a hot wire. The last guy should've made red and black the travelers and tagged white with colored tape in the segment where it served as switched-hot. That would've been reasonable.