Inspiration struck after spending the evening ga(y)zing at Jeffrey Hunter as Captain Pike in "The Cage" (he's so dreamy) - specifically, his
excursion jacket. The jacket has a wide panel running down its centre which matches the grey panel of the
Voyager flight suits, plus it has pockets (a rarity in Starfleet uniforms) which seem eminently practical in such a garment. The colours will be based on those of TOS - a gold centre panel with darker edging (and, possibly, silver-white piping as a nod to the bands around the sleeves of the TOS excursion jacket and the white shoulder yoke of the Voyager flight suits), with a navy blue/grey or dark grey/black main body (Memory Alpha states the jacket to be grey, but my colourblind eyes see blue).
So, my TOS flight suit will look something like this, I hope (please ignore the toast crumbs - I was eating breakfast when I took the photo. Oh, and ignore the waste extraction bit - that was my justification for having primary coloured tubes running over it a la TOS
environmental suit, but I've given them the heave-ho now.):