It may be Blue or White, nothing else to begin with (4)
I believe the answer is:
nile
'it may be blue or white' is the definition.
(Blue Nile or White Nile, tributaries of the Nile)
'nothing else to begin with' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'to begin with' suggests taking the first letters.
The initial letter of 'else' is 'e'.
'nil'+'e'='NILE'
(Other definitions for nile that I've seen before include "River associated with Cleopatra" , "'The longest river, blue and white (4)'" , "World's longest river" , "River at Luxor" , "Current" .)