This is the base for a commission I'm working on presently. Happy how it's turned out.
Does the colour of the helmet look right? And the colour of the jacket?

Nice start.. The colours seem right.. but I will wait for those more qualified than me to confirm.

Early helmets could be green or a mottled finish of various colours (not bright patches though). Fairy quickly these finishes were replaced with an overall less-reflective khaki finish. Here's an example of an allegedly original early helmet: https://www.iwm.org.uk/collections/item/object/30091017

As for the jacket, you can use a wide range of khaki through to greenish drab colours - if you have Mike Butler's book, there are good examples of original uniforms that show a wide variety of colours. And if you don't have Mike's book, I recommend getting it The colour you've used looks fine.