There exists a copy of some draft requirements that the BBC had for the new machine. At the time, these had already been hashed out a bit with Acorn, so they match quite well with the resulting BBC Micro. A relevant extract:
Keyboard: capable of generating all 128 ASCII codes.
Positive action keys (not touch sensitive).
ISO standard layout plus:
(a) Up/down/left/right cursor control
(b) A row of keys, above the
numbers, which generate software definable codes - this could be done
with a software look-up table to map the original codes to new values.
There is no mention here of keycap colours. On the actual machine, the cursor editing keys (four arrows and
COPY) are olive green, the ten function keys are red, and the rest are black.
The result is instantly recognisable, but most likely arose from ergonomic considerations at a relatively late stage of design.