Umedaman said:
upto the 1990's, clubs rarely had use for a 3rd kit, in fact mostly, there were only home and away kits, with the away kit classed as an 'alternative' kit, incase the home shirt clashed with the team they were playing away from home, so the alternative kit was used.
a 3rdkit was introduced in case the 1st and 2nd kit still clashed with the home kit when playing away from home, but this was quickly used as a commercial tool to generate revenue to offer more range for replica kits.
the alternative 2nd kit or 'away' kit normally used to use follow a traditional 2nd kit. in the case of City, this might have been a white shirt with blue shorts. but this has changed with the likes of the black/red stripe kit (introduced by a manager at the time to resemble the mighty AC Milan at the time) ... so there are many varieties of an alternate kit over the years.
but mostly now the colours get rotated between the away kits and '3rd kit' with the home kit (in theory) remaining the same.
as in "how do clubs decide on designs", we always used to kit that manufacturers gave the clubs a list of designs and the clubs chose what they liked. but in the modern day, multi-million deals that manufacturers pay to clubs for, it will be the manufacturers that decide on the designs (staying within a pre-agreed scope), as they are the experts and know more about "what sells more" (in theory) .... at least .... the manufacturer will strongly advise what the kits should look like etc ... but its all in the details of each club deal who decides what.