Mother of Groom

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And here are some more

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This kind of thing?
I'd be happy in something like this.

You sound like a relaxed dresser, so even as MOTG I don’t think you need be too formal.I would wear a smart green and black patterned top and black trousers for the RO official bit and a more informal look for the woodland bit, a maxi or midi dress in green with comfortable sandals.
Unusual choice of wedding colours😁

You said green was a theme colour, how about something like this for the register Office. Phase Eight

I agree that trousers and top/jacket would be appropriate for both. If you wore some wide trousers and a more formal top to the registry office, you could wear the same trousers and a different, more party ish top to the woodland ceremony.
We went to something similar and I wore palazzo trousers with a long blouse/ jacket over it. I changed my pink suede shoes for trainers later!

Honestly, no, I don’t think trousers and a non matching top would be a good choice for mother of the bridegroom - trousers are fine, but they need to be part of a set in my opinion, either a trouser suit, or matching top and trousers, there are some really nice matching sets around this summer, some have trousers with a border which matches the top.

I agree trousers and top and as it’s a woodland shoes that you are comfortable in is fine. I was at a very informal wedding this last weekend and some wore suits and ladies pretty dresses but others wore trousers with tops or jackets one lady had skirt with boots, my thoughts were people will be looking at the happy couple not me. Have a lovely time just be comfortable and you’ll be fine.

Something semi-formal for the Register Office, anything you feel comfortably smart in, but more casual for the Saturday. Black trousers, green coloured top?

How about a trouser suit with pretty but not necessarily frilly blouse. Sounds like you know the brides mother could you find out what she is wearing. Perhaps go towards the green rather than the black. Hope the weather is kind!