There’s been a lot of debate about this over the last few years. The situation is also made more confusing by the fact that most regiments were issued different coloured coats with each issue (eg, one year you might be green coats, the next year grey coats etc). Many locally raised regiments (where we often have only a fleeting reference) only lasted for one campaign before being disbanded (plus where a coat is issued it is meant to be worn over your normal clothes). You then have to remember there are a lot of very tentative “theories” associated with the ECW (eg all Newcastle’s Army had white coats and scots bonnets, all Cornish troops wore blue with stripes etc etc. There is very little evidence supporting these “accepted facts”). Remember as well, when you read of a red or blue regiment etc that most often relates to the flag colour and not the coat.
If you are fielding one of the main field armies at the start of a campaign quite a few would have issued coats, by the end, very few.
Best guess is that possibly 2/3 of regiments could be “non-uniformed“ but that literally is a guess. Field signs were worn for a reason!
Dags has done a brilliant job really catching the feel of an ECW Battalia here...