Source of colour against capturing quantity of light (6)
I believe the answer is:
crayon
'source of colour' is the definition.
(I've seen this before)
'against capturing quantity of light' is the wordplay.
'against' becomes 'con' (as in pros and cons, for and against).
'capturing' means one lot of letters goes inside another.
'quantity of light' becomes 'ray' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'con' going around 'ray' is 'CRAYON'.
Can you help me to learn more?
(Other definitions for crayon that I've seen before include "Writer" , "Wax pen" , "Drawing-stick" , "Waxy pencil" , "pencil (13) [FRENCH]" .)