An explanation of one thing with optional interpretations.
The count requesting a girl 'as white as snow' could suggest he is seeking someone innocent and vulnerable, demonstrated through the use of the colour 'white' which connotes purity. This may link to ideas surrounding purity such as that of a young virtuous girl, sought after for her virginity, as explored in the Madonna whore complex - men desiring women for their cleanliness.
However it could also arguably indicate the count has become unsatisfied with his wife, as she wears 'felts of black foxes' and is seemingly the opposite to the white, wholesome figure which the count conjures demonstrated within the direct contrast of their characterisation. ( It could also suggest within this idea that men desire women without personality, and merely to satisfy their sexual appetite, something featured within a number of Carter's short stories).
However another interpretation and one that I feel is perhaps the most powerful is the use of the bold colours and their visual elements which depict the proposal of the 'male gaze'. The count; the male character conjures up a girl only using visual references to her appearance using colours, illustrating his care in appearance but his disinterest in personality, suggesting he seeks the girl merely for her exterior. She is seemingly objectified as a female but more as an idea rather than a character, an illusion of a generalised male desire.