replace deepest level of nested list

[David Isaac]

I have a list of lists, N+1 deep.
Like this (for N=2):
[[['r00','g00','b00'],['r01','g01','b01']],[['r10','g10','b10'],['r11','g11'
,'b11']]]

I want to efficiently produce the same structure
except that the utlimate lists are replaced by a chosen (by index) item.
E.g.,
[['r00','r01'],['r10','r11']]

N is not known ahead of time.

Suggestions?

Thanks,
Alan Isaac