Ok. The Reiss dress you've just posted is very nice, & you would certainly look like a stylish aunt. However, even though it's probably a good colour for you, I do think it's on the sludgier side of the green spectrum. If it was satin or silk, it would be better, but I would say it's rather a muted dull green & not "colourful" by most standards. Nor, as it's not in a luxury/embellished fabric, is it "cocktail".
Also, although it wouldn't be as short on you (the model is 5' 10"), the tie detail at the back reaches almost to her ankles. I think it might look a bit silly on you if it's touching the floor??
I have a feeling that you have a preference for plain muted things in general (as so far the dresses you've liked best have been a long beige/brown crochet one, a long plain gold one, & a short plain sludge-green one!!!)?? Is your preference for a plainer dress that you accessorise to dress it up? You said you dislike patterns - what about beading/sequins/embroidery? As it's generally either those sorts of details, or a more glamorous fabric like velvet or satin/silk/gauze/tulle etc, or both fabric AND embellishments, that make the outfit appropriate for such a code?
I know that pink won't be your colour, but to give an example of a plain dress also by Reiss that I think could be made to work by virtue of it being colourful, with glam accessories, it's this very similar one?