In article , Dave Poole
writes

snip
In most plants, chlorophyll completely masks other
pigments during active growth.

snip
With the reduction in nutrients, the chlorophyll breaks down and this
enables the previously masked carotene and/or anthocyanin pigments to
show through.

Good. That confirms what I deduced from observation, particularly with
maples - during the season the so-called red leaved maples are in fact
brown, which is a mixture of green and red, but go a much clearer red in
autumn as the green is removed. It can be seen very clearly as sweet
peppers ripen, from green through brown to red.

In some plants, the anthocyanin levels are very high as
the result of a secondary action of simple sugars in the presence of
sunlight (which is why fruits ripen red, orange or purple on the sunny
side). Plants with increased levels of anthocyanins in their leaves
turn red or reddish purple when the chlorophyll has broken down.

Are anthocyanins the same batch as you get in all those flowers which
are not entiely sure whether they are going to be pink or blue - like
lungwort, forget-me-not, cornflower or geranium?

Not
all tree carry anthocyanins though - the birches, beech and ash only
have carotins.

And therefore have an autumn colour of yellow?


They do here as well, - Aesculus are the first to colour up will start
even in August if the weather is very dry. Conversely the 'Turkey
Oak' - Quercus cerris holds its leaves until early December and
suddenly turns dark brown within a week, shedding everything a week
later.

the latest I've found is Swamp cypress, which here is still green and
hangs on to its leaves so late that I keep thinking it's going to stay
green all the year through.


--
Kay
"Do not insult the crocodile until you have crossed the river"