oh dear that is not looking too happy.
Has it been grown supported by other plants in shade? Does the pot have good drainage?

I wonder if there is anything nibbling the roots. Has it just recently gone floppy? Carefully knock it out of the pot and look for vine weevil grubs.

as for them not being the colour you ordered I'd get back in touch with the company.

Have they been grown in a greenhouse? They may not be acclimated to the outdoors. Whatever the case, these plants didn't get sufficient sun. This causes stem elongation and the plant cannot support its own weight. I don't think anything can be done to fix it at this point. Those stems are too flimsy to support even leaves - never mind flowers. The stems should be thick and turgid, like celery. Of course stressed plants are more susceptible to other problems so yes, check the roots. Send these back!

Getcha money back these plants are defective. You can still get dahlia tubers that you can plant yourself. They are late summer/fall bloomers so it's not too late. I'm going to post pictures of my dahlias right now so you can see what they should look like.

I bought a packet of Sarah Raven cosmos seeds and only one seed germinated in a greenhouse seed tray - the way I have successfully grown cosmos in the past.

I'm often tempted by Ms Raven's £5 off vouchers. I tend to think 'well that's the postage sorted' but I must say that their plants are variable even though expensive and I expect the best. They are good at refunds or replacements though.