CST is defined by destination address in BPDU frames which is 01-80-c2-00-00-00 (IEEE 802.1d). The reason is compatibility with non-Cisco devices.
Cisco PVST+ approach is:
Each VLAN BPDUs are sent to 01-00-0c-cc-cc-cd plus VLAN1 BPDUs are copied to 01-80-c2-00-00-00 on trunks.
BPDUs are sent to 01-80-c2-00-00-00 on access ports (no matter to which VLAN the port belongs - this is not described in Cisco documentation).
So if you put your access ports to VLAN2 they are a part of VLAN2 STP topology (together with the possible non-Cisco switches connected to these ports). You can even connect new Cisco switches via ports assigned to VLAN3 to these ports and the new switches will become a part of one common VLAN2-VLAN3 STP topology. (You will receive a VLAN mismatch errors via CDP but STP will be still working.)
I know this is pretty confusing but it's necessary to understand that Per-VLAN-Spanning-Tree is meant LOCALLY on a particular switch and port-to-VLAN assignment is also local, so there is possible following spanning tree topology creating one (common) STP tree, e.g.:
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