Existing thermostat wire colors - no white?

Question

Looking to switch from an old Honeywell CT87A thermostat to a smart thermostat, probably something requiring a common wire. There are 2 thermostats (one on each floor) in a 2 zone forced hot water oil burner system, heat only.

I want to make sure I have the right wires before buying a new thermostat, and was worried with an old system like this I'd have just 2 wires. But looking at the existing wiring, there are 4: red, black green and yellow.

• Red and Black wires are on the thermostat's R terminal,
• Green and Yellow on the W terminal
• nothing on the Y terminal

Looking up thermostat wire color charts, I am now very confused as I have no White wire which is pretty much always shown as being for heat.

Can I assume either my Green or my Yellow is actually for heat, since they are both connected to the W terminal?

Assuming my Red is for power (since that's consistent with 99% of color guides, and it is connected to the R terminal), is my black wire a common?

Can the wire roles be deduced in this situation just from the terminals they're connected to? Is there a way to find out for sure?

Thermostat on 1st floor:

Thermostat on 2nd floor:

On the boiler end, there's only one thermostat-gauged cable coming from the junction box (red arrow):

And that one cable goes from the junction box to the following taco box, and has just black and white wires:

Answer 1

Someone used telephone wire (2-pair red/green & black/yellow) for the thermostat wire here.

So, ignore anything you may think applies about "thermostat wire colors" and just pay attention to what is connected to what.

In this case, they have joined the pairs, so it's essentially acting like red/white "thermostat" wiring. Red/Black is "Red" and Green/Yellow is "White" as seen on your first device.

Telephone wiring is normally smaller gauge than thermostat wiring, so it may prove inadequate for powering your "smart" thermostat if the pairs are separated into 4 individual wires. Sounds like that won't matter anyway as you only have actual red/white from the boiler, so you'd need to upgrade all the thermostat wiring.

Or manage to deprogram yourself from the marketing mayhem that convinces people they "need" a "smart" thermostat, but evidently that's some major ju-ju of diabolical marketing genius.