If you open the link below and scroll down to the "MAKE IT YOURS" section you can see how BRG with roulette wheels and yellow caps looks:
THE ALL-ELECTRIC MINI | MINI CY
To my eyes it looks completely stock/factory, the yellow on the front & back badge and wheel caps complements nicely the yellow on the mirror caps.
The only reason why I went black mirror caps on my BRG is because I am old and prefer not to draw attention on the road.
In addition, yellow mirror caps (in my mind) also advertise the fact that this is an EV and I prefer the look of a "standard" ICE car (drivers do not expect the quick acceleration of the EV combined with zero engine noise which I also prefer).
If I was younger I would definitely go for yellow, I believe it lifts the whole car from boring looking to exciting (akin to silver mirrors on past Audi RS models), plus I hate the fact that black always looks dirty and scratched.
It is good to ask for opinions but in the end you should get what you want, this is not an irreversible mistake, it is just 2 not very expensive mirror caps which MINI sells different ones anyway if you want to change them (mirror jack style/etc.)