Why have the leaves on my Japanese Maple tree lost some of their redness this year

That's a nice mature specimen. The leaf colour can vary slightly for all sorts of environmental reasons - not enough water, not enough sun, too much sun, masses of water available. I'd say conditions this summer, well in the south anyway, have been much drier generally and also warmer, on some occasions, hot, compared to last year. Dryness at the roots can cause the leaves to look "faded".

The leaves on our acers are looking very greenish this year - which is a first... I just put it down to the weather

that,s a beauty..

Give it some liquid ericaceous feed and that might perk it up a bit

Andrearichter it would be difficult to feed an acer the size of Janer's. Given that the leaves will drop off within the next few weeks I'd just let it go to sleep for winter.

I beg to differ as there are products that you can add to your hose that provide a liquid feed. It is not too late in the season, especially as we have had a dry spell. The product i'm talking about is Miraclegrow. I know they do a general feed and ericaceous, but the screw on hose application may only come in the blue, which is general. I did this for a camelia and rhody, I just put the hose on lock and left it on the ground a tthe base and it did the job for me. I had a spare container and I filled it with the pink soluble feed which is the ericaceous.

Not a good idea to feed any plant (except winter veg and annuals) past the end of July, and certainly not now, it encourages soft and sappy growth, which is the last thing you need as the winter is hurtling towards us.

Thank you Bamboo...

Hey, that rhymes, moongrower;-)

Do it in Spring then!!

my acer has been the same, more brown than red this year, but healthy enough