Colours of old Sierras (7)
I believe the answer is:
oranges
'colours' is the definition.
(orange is a kind of colour)
'old sierras' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sierras' becomes 'ranges' (I can't explain this - if you can you should believe this answer much more).
'o'+'ranges'='ORANGES'
'of' is the link.
Can you help me to learn more?
(Other definitions for oranges that I've seen before include "a zesty lot" , "Citrus fruits" , "Gear son up for fruit" , "They are not the only fruit, wrote Jeanette Winterson" , "Navel, blood and Seville are all varieties of these" .)