"If they are not colorized, then label them with pieces of paper and tape."
How you do that is look at the terminals where the wire is connected. You should see a notation near there (sometimes you may have to look under the wire to see it).
What you should see are colors, or codes for colors. Normal ones are:
C-common (usually black wire)
You might also see more than one, especially with White & Yellow. These are then normally marked Y1, Y2, etc. (But Y2 probably isn't a yellow wire, but some other color wire that is in the cable.)
On newer systems you may also see additional wires:
You might also see other colors, like Pink, Brown, Gray, but those are normally marked by various function codes rather than color. And they will likely vary by manufacturer.
One quick guide is to count the number of wires in the cable between the furnace & the thermostat. Really old furnaces will have only 3 wires (Black, Red, White). Newer ones, with Air Conditioning (or plans to add it) will have 5-wire cable, with Yellow & Green added. Additional wires indicate a newer system with multi-zone systems, heat pumps, or multi-stage furnaces.
And one thing to watch for is that old-time contractors sometimes used whatever piece of cable they happened to have around, ignoring the color code. Not uncommon to see old phone cable (red, green, black, yellow) used with red & black as normal, either yellow or green used for the white wire, and the other just unused.