How well do my groomsmen/bridal party ties need to match my bridesmaid dresses?

The time of year may be a factor but I see its May so that will not have an effect here. I am surprised that you are struggling to find the right shade but it is a very common issue and people usually go with as near as they can get and on the day and on the photos no one really notices.

Are the men in waistcoats so there will not be much tie showing ?.

As the tie is darker you may be better off going the other way so the tie is lighter if you can not find a good match because there is a lot of the green dress and a slither of green tie which will also be on a white background (shirt ) possibly making it look darker.

But even if you find an exact match in a flat light the tie material may look different in different light. And then you go to all that trouble and your photographer uses a burnt out washed out "look" or a dark orange look. Popular right now is the desaturated green look. So if your photographer uses any of these and you are wasting your time with trying to colour match I'm afraid.

I don't think the colour difference matters at all - the two shades don't look weird even right next to each other, and bearing in mind they are going to be worn by different people (and the ties aren't that large) it will matter even less. Yes, people may notice that the ties and dresses aren't the exact same fabric, but they probably wont' be expecting them to be anyway.

Hi Olivia, if you did want to go down the colour matching still (although great advice from the last few responses) have a look at a company called Swagger and Swoon, they have a great colour matching service plus you can order samples for ties/pocket squares etc. If you speak to them first and let them know the colour or your struggles they are really accommodating and may send other suggested colours too. x