what would be the best practice to achieve separate tree topologies for each of the rings?
Forget the rings and reconnect the switches in a tree topology with one or two center switches.
This double-rooted tree topology has no single point of failure. Any access switch failure breaks connectivity only for hosts connected to that single switch. Any host connecting to two access switches has fully redundant connectivity.
A tree is a more natural way to arrange an Ethernet network. It keeps paths shorter, removes bottlenecks, has a smaller diameter and provides better resilience. Of course, you can forgo redundancy and use a single core and single uplinks. Even a simple tree has just the core switch as single point of failure. Your ring system has two SPoFs (the outer switches in the light red box).
In any case, make sure your root bridge is always well-defined (one of the center switches).
I do not completely understand IF RSTP can be made VLAN-aware
RSTP is entirely VLAN-agnostic and works on the port level only.
You can use MSTP with multiple instances and grouped VLANs to overcome that limitation. If you must stay with the rings you could use instances to optimize link usage, but note that an instance border is never in the middle of a link (as pictured) but inside a switch (between VLANs).
Note that the topology in your diagram exceeds the design limits of RSTP and single-region MSTP (maximum diameter of seven bridges) and will become instable if the 'wrong' node fails (the diameter prevents proper convergence). You could overcome that limit by running multiple MSTP regions.
All in all, everything is going to be much simpler in a proper tree topology. Also note that the leftmost and the rightmost RING-B switches are single points of failure, cutting off the lateral rings.
The double-rooted tree pictured above has very short paths and no need for configuring multiple MSTP instances or regions. Optionally, you could optimize link usage by configuring MSTP instances, then it's also better performing than the chain-of-rings topology in your question. Of course, IEEE 802.1aq Shortest Path Bridging is almost always superior to xSTP.