Whatever the colour they always turn out red except if they’re white then they fire generally white or off white. I always have a trowel and plastic bag on my travels just in case I see some being dug for pipes etc. The green clay is from Friars cliff in Dorset and is probably from the Wealden clay group associated with Greensand ( sounds like I know what i’m talking about ) the grey clay is from Folkestone,this falls out of the cliff and is washed away into the sea.This is Gault clay, it has all sorts in it, loads of fossil shells,gypsum all shiney and sparkly and shrapnel from ww 2. The yellow clay in blagged from a brickworks in Kent and is lovely to work with,no results yet . This is called Kent clay. My house sits on clay, when they dug the footings for a new garage at no.42 there were skip loads of it,this too was yellow but quite hard to use, I made a little pizza oven with it but is barely holding up, this clay would probably be only good for cobb building.
There are many types and colours of clay in England, why not put a trowel and bag in your boot (make sure you’ve got a sock on ) and keep a look out.