Before the killing frost hits, many plants show a lot of red before death or dormancy, and some will show it even after. Sometimes the red has no special value, and it’s just the result of the plant turning off a function, like the leaves of this woodbine.
Japanese maples often turn red once they stop producing chlorophyll before dropping their leaves.
This is an oakleaf hydrangea.
Pokeweed stems are reddish purple all year but become more prominent as the leaves die. Since pokeweed is poisonous to many animals, it’s possible it’s just a permanent adaptation that acts as protection from being eaten, rather than a seasonal adaptation.
Roses aren’t unique to fall, of course, but some varieties will keep blooming until frost, and their red color is just advertising to any pollinator still hanging around.
However, in this season many plants will produce red fruit or seeds to attract birds, who can see color, and help them spread. This amaranth turns completely red, seeds and plant alike.
Other examples are this viburnum
and the well known holly berries.
I get the feeling that a lot of these aren’t particularly well liked by birds — they’ll often last well into the winter when there isn’t a lot of food around, until birds finally have no other choice, it seems. Or maybe the fruit ripens really slowly and produces sugar when the seeds are fully developed? I can’t say I’ll ever try them to find out.