Will this look just really weird and out of place?

It's hard to imagine it all together, do you have some pics of individual bits we can see? In my head it looks fine, just because other stuff is paler doesn't mean you can't have something nice and bright in your centrepieces. I love sunflowers and personally think they're rustic and fit in with a country garden feel x

If I'm honest I think if you are going for pastel colours then sunflowers will look too bold...they are a very 'statement' flower and in my opinion work better with plain colour schemes or autumn themes....why not try things like hydrangeas that will fill the jugs? X

I think you are right pastels will fit better.. I did look in to hydrangeas but they bruise and die very easily x

Sounds lovely to me. The only downside is that getting them can be tricky - a lot of florists near me don't ever have them or only have them for a very short time. I love freesias, especially the smell! But they often aren't that tall so with short stems, they might look a bit swamped in the jugs....Maybe a few long stemmed roses mixed in with the sunflowers would tie it all together? (I think all flowers look pretty and I like things to be varied rather than all matching, but I appreciate that a lot of people like things to all be within a colour scheme. I'm going to have whatever flowers are in! Saves planning and worrying and keeps things cheaper!)

I'll be having sunflowers, but it's an autumn wedding and I'm going quite rustic with neutral colours.

Deffo stick with pastel colours in the centrepieces I think - have you considered silk or foam flowers?

Sunflowers and rustic can go really well together https://ohsobeautifulpaper.com/2009/08/real-invitations-jennifer-james/

Or what about some giant daisies? https://www.123rf.com/photo_9798077_white-oxeye-daisies-in-a-turquoise-enamel-jug.html and https://www.gapphotos.com/imagedetails.asp?imageno=258388

To be honest, I think it the sunflowers would look out of place.

I have one daughter getting married in 9 days time and she is having a vintage/rustic style wedding. Colours are dusky pink, lilac etc and sounds very similar to yours ie enamel jugs on table. Her flowers are roses. lisianthus, thistles, stocks, daisys and other similar flowers. For the table flowers could you not add in other less expensive flowers. The florist did a mock up of the jugs for us and only had one or two roses in, yet still looked stunning.

The other daughter wants sunflowers at her wedding, but a mix of her wedding style and the other daughters would look a bit odd. Sunflowers are a very bold statement and everything else would need to work with that.

I went for the same style of wedding - shabby chic/country, and i had sunflowers. they looked lovely, however, they werent normal sized sunflowers, they were much smaller, and i didnt just have them in my centre pieces, i had a sunflower/country bouquet, sunflower wedding cake, sunflower umbrella (we got married in the lakes), yellow wedding shoes and wellies to match and the men had yellow socks on

Carnations last ages, and so does Gypsophilia, and although they are cheap they give a lovely frothy look. I use them at work a lot for our displays.

I would stick with the original plan. I don't think it will look too perfect, instead it will help tie all the elements together and help strengthen the theme.

Yellow can look very nice for rustic weddings, I'v recently had a wedding submission accepted over at whimsical wonderland weddings that had a yellow and blue colour theme, but in this case I think it would clash with your current colour palette.