What 3 colours are referenced in Shakespeare's poem? What do they symbolize?Sonnet 12 by Shakespeare.

This sonnet is about the inevitable passage of time.  In
the octet (the first 8 lines of the sonnet) the speaker uses several images to convey
the reality that everything that is young, alive, and vibrant, will fade away and
eventually die.  He uses color imagery to make his point more
clear. 

The opening line of the poem establishes the
subject:  the clock to always moving forward.  From there the speaker seems to look
around him for the evidence of the passing of time.  He notices that the bright sun of
the day time fades to a "violet past prime."  He is describing that
dark blue/purple sky just before the sky becomes black at night.  There is just a
suggestion of light left.  The life of day has faded to the death of
night.

The next line compares the sable
curls of his hair (or someone's hair.)  Sable hair is a rich dark brown or
black.  It the hair of a young person.  As that person ages, the hair gradually is taken
over by (silver'd o're) by the white hair of old
age.

The next three lines talk about the passage of the
seasons.  He describes the green of summer when the trees provided
a canopy, and contrasts that with the now barren trees that lack color and are instead
are covered in a "white and bristly beard" of frost or snow. The
season has passed from the life of summer to the dead of
winter.

He makes his ultimate point in the final sx lines
(sextet) of the the sonnet.  He states that everything that is beautiful must eventually
die -- days, hair (a person), seasons. Nothing can stop this natural cycle, but having
children will at least allow the natural beauty of a person to continue (not die
completely) as that child will carry on the beauty of the present day
person.