The reality is, the clothing industry decides what to produce at least 6-12 months in advance. It's not like they produce everything and public opinion decides in the end what is "cool". Granted, the industry tries to predict what will be in demand. But by and large, fashion is not predicting what will be cool, but specifing what will be produced.
When they say "blue will be in fashion next year", then it means you'll be able to buy a lot of blue clothes (and some people will be able to deduce that you have not recently bought clothes, which is a bonus to encourage more buying).
I only realized this a I got older, when certain items were missing in every shop, that were there the previous and following years. Be it a certain cut of jeans or a certain colored shirt. (And it's not just clothes, I noticed it in interior design, PC cases, salad sauces, and just about any product category.)