I have an 8 watch box. My main rule is simply: one in, one out.
Other than that, I've limited myself to around 3k max in one watch.
I try to get watches that feel different enough from each other, although it's sometimes hard because I have somewhat specific and boring taste (black, blue, cream, silver, dials)
The current collection is:
Tudor BB58
Nomos Orion
Sinn 104
Halios Seaforth
Formex Essence
Monchard pulsation Chronograph
GS SBGW231
So I have one spot left and a few ideas, but nothing definitive.
There's nothing wrong with prefering black and white. I'm the same as you.
Only color I have is a BB58 in blue.
I'm tempted to get something in green, like the Nomos Club sport in petrol or maybe an Omega AT in green.