One's in black or conversely one may come in blue (4)
I believe the answer is:
biro
'conversely one may come in blue' is the definition.
I can't judge whether this defines the answer.
'one's in black or' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'or' becomes 'ro' (I am not sure about this - if you are sure you should believe this answer much more).
'b'+'ro'='bro'
'i' inserted within 'bro' is 'BIRO'.
Can you help me to learn more?
(Other definitions for biro that I've seen before include "writer" , "Lazlo -, ballpoint pen inventor" , "Pocket item" , "Type of ballpoint pen" , "Writing implement" .)