I do have quite a large fabric stash, and quite a lot of it was bought deliberately with SWAP type wardrobes in mind, only I didn't really follow through and sew them all up at the time. So I can often find co-ordinating fabrics when the muse strikes me. This is good :-) but does require storage space. I am currently on a fabric diet, and really trying to not buy anything new. This is fairly easily done by not going near fabric shops, which is fairly doable in my normal life.
I want to sew a collection I think about the main colours - normally a
base and a colour - e.g. black and red or brown and orange. I then go
off into the stash and pull more pieces than I need which match those
colours, refine it a bit later in daylight when I can see if things
really 'go', and put some of them back. I then sew up a few
co-ordinating garments without changing the thread in the machines.
plan often morphs as it goes along, and recently I've been doing
several garments from the same fabric, rather than putting small remants
back, but that may just be a phase.
I am definitely driven by the fabric, which then dictates the pattern, I very rarely work the other way round interestingly.