How can I get my red Japanese Maple which has gone green to turn back to red?

Not sure that too much nitrogen would cause this. You don't say whether its in a pot or the ground, and also whether its in sun or total shade. Most Acers, if left in complete shade, will lose their leaf colour. On the other hand, if left out in hot sunshine in the middle of the day, they fry and the leaves go crispy. so what I'm saying is, they need some sun, just not hot sun. Your tree is about to lose its leaves now anyway, but you might want to consider changing its position if it is in total shade.

It may help to give it (in Spring) a dressing of Sequestrine.. I don't know where I got this from, but I do it a couple of times a year to maintain the colour.

Mad - there is a belief (erroneous) that Acers like acid - its nearly right, they don't like chalk, which is not quite the same thing, so if your soil is alkaline and chalky, an acid dressing will help it.

mine has turned green from red think its just that time of year maybe

x x x

Thanks Bamboo. I should test mine really, but it may well be alkaline because my Hydrangea is very pink and definitely needs sequestrine just to keep going (and ericaceous mulch).

Our Acers are looking a lot greener than they did earlier in the season. They will all be dropping their leaves soon so no worries.