The use of the preposition "against" in the sentence you mentioned suggests that the color of the subject in question provides a protective measure against the winter snow, rather than blending in or being associated with the snow. Using "with" would suggest a more general association or connection between the color and the winter snow, without necessarily implying any particular function. Using "in" would suggest that the subject is actually located within or surrounded by the winter snow, which may not necessarily be the case. Therefore, "against" is used to convey the idea that the color is a defense mechanism that helps the subject to blend in and avoid being easily spotted by predators in the snowy environment.

All of those words work. As a verb, "camouflage" means a sort of combination of "disguise", "conceal", "hide", and "deceive". You could also use: "beside the winter snow" "within the winter snow" "before the winter snow" You could even say "among the winter snow" It's hard to go wrong. I find the last one rather poetic.

Yes, I agree with you. It's a bit strange. One of the ways in which we use the word 'against' is to mean "in visual contrast to" This is often used to describe paintings. For example "Rembrandt is known for painting figures of people against dark backgrounds." You can see certain parts of the figures better because of the background colour. In other words the complete opposite of what camouflage should do. One is camouflaged against detection. Not against the background. So I wonder if perhaps the person who said/wrote this was confused and said the wrong thing? I would say "in the winter snow" (though I think that the word 'winter' is a bit redundant here)