Operating in colour that’s extra strong (10)
I believe the answer is:
reinforced
'extra strong' is the definition.
(reinforced and given extra strength)
'operating in colour' is the wordplay.
'operating' becomes 'in force' (in force can mean active or operating).
'in' is an insertion indicator.
'colour' becomes 'red' (red is a kind of colour).
'inforce' going into 'red' is 'REINFORCED'.
'that's' is the link.
(Other definitions for reinforced that I've seen before include "Underlined" , "Added to" , "Strengthened" , "Sort of concrete" .)