What is the correct color of yagnopavitam?
None of the Smritis that i have read (except UsAna Smriti ) mention colors. Only thing they mention is the material which the threads are to be made of. And, these materials do vary with varnas.
Manu Smriti 2.44. The sacrificial string of a Brahmana shall be made
of cotton, (shall be) twisted to the right, (and consist) of three
threads, that of a Kshatriya of hempen threads, (and) that of a Vaisya
of woollen threads.
And, the same is stated in other Smritis as well.
The only Smriti that talks about a color is the UsAna Smriti (which is one of the 18 major Smritis as per the Yajnavalkya Smriti):
Formerly, cotton-made sacred thread was instituted by Brahma.
Strings of three threads are for the A Brahmanas. hempen cord for a [
Kshatriya] and ; a woolen one, a [for Vais'ya]. (6)
A twice-born one should always wear the sacred thread and tie up the
tuft of hair on his crown. He should put on a most excellent white
piece of cotton or silk cloth without any hole, but quite different
from* the one used before.
UsAna Smriti's Chapter 1.
Note that, what it says is formerly (that is prior to UsAna's time) the sacred threads were made for different varnas out of different materials. But now (that is during UsAna's time) the threads for all Dvija varnas are to be made of white cotton/silk only. Now, this makes lot of sense, because nowadays everyone use (mostly) white cotton threads only, irrespective of varnas.