In diamonds I go for blue colour (6)
I believe the answer is:
indigo
'blue colour' is the definition.
'indigo' can be an answer for 'colour' (indigo is a kind of colour). I'm not sure about the 'blue' bit.
'in diamonds i go' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'in'+'d'+'i'+'go'='INDIGO'
'for' is the link.
Can you help me to learn more?
(Other definitions for indigo that I've seen before include "Blue-violet colour" , "Deep dark blue" , "Deep violet-blue colour" , "Violet-blue dye" , "Blue dyestuff" .)