The word varna comes from the dhātū , the root verb √vr which means ‘to cover’. See “ Nirukta “  for the English version and “ inaÉ>ma ”  for the Hindi version. The passage describing varna includes the description of kalyān – varna – rūp threesome. Here kalyān is referred to the brilliance of shining-gold, varna – that which covers and rūp as beauty and its form. A later and more detailed explanation comes from “Amarakösa”  :-
varno dvijādau śuklādau stutau varnamtu va akśare.
aruno bhaskare api syāt+varna+bhede+api ca trsu .
varnah has two lineages viz. white etc. colours and stuti meaning eulogy, singing praise, panegyric. Thus it means both delineation of colours as well as delineation of sounds or varnam the letters, phonemes.
arunah is the name for the sun, for its red hue as it rises ; perhaps as a third meaning varna is the separation of the suns energies or rays.
Thus varna is expressed in the modern dhātukösh , dictionary of root verbs, as both varne and āvarne. The former means to choose, to limit ; while the latter means to cover, to hide.