Because the 'seasons' on Instar are so long, the fashionability of certain things, like the colour of clothing, is unrelated to the time of year. This applies somewhat to style of clothing too, although short-sleeved shirts are never going to catch on in the depths of winter.
Almost all clothing is bought bespoke, or at least altered. Mass production is anathema to right-thinking people and off-the-peg clothing is not something in which anyone who can afford otherwise would be caught dead.
Bright colours are the order of the day. Dull colours are never sought-after except for strictly practical purposes such as camouflage.
Certain colours are deemed to be in fashion at any time, generally two colours at once. It is generally clothing manufacturers, and to a lesser extent celebrities, who set the trend. Black and grey are not colours in the strict sense of the word and are used for accents, etc.
Only one person in the entirety of Instar is permitted to wear white.
Although beauty standards do not tend to change much, certain traits in a boyfriend, mistress, lover etc. will be fashionable at any time.
If you're married to someone whose particular colouration fall out of favour, it will be understood that your spouse was fashionable at the time, the match is still perfectly respectable and nobody can do anything about it now.