Hi Aryan, what have you tried so far? You can achieve a lot with our rules and actions system, sometimes it takes some experimenting to get the result you are looking for if you are new.
I am going to use this setup as an example.
I created two rules:
Essentially it says: if Paint is any of Paint A and has changed, set the Subitem value to the default value I want (in this case Paint A - Subitem 1).
Recording of it in action here.
Let me know if this is helpful or not!