@BunTooti IMO you are not being unreasonable at all. It stinks.
It may be right, and the seller may be within their rights to make you send the red jumper back first before sending the correct (blue) one to you (I am not 100% sure,) but it still stinks. It's THEIR mistake. Why the F should you be inconvenienced? 
I have had the wrong thing sent by 2 'etsy' users this past few weeks, (just £3-4 worth - but still the wrong item.) I told them, and sent a photo of the one they'd sent, and they both apologised and sent me the one I had ordered, and told me to keep the other one!
However, I ordered something that cost £15 from ebay 3 weeks ago, (eg, a green dress,) and they sent a purple one. I messaged them and told them, and got the response 'both colours look good - you can keep the purple one,' and refused to admit their mistake, (saying I had to suck it up basically.)
I said 'I don't WANT the purple dress.' I was told then that I had to send the purple one back, and they had to receive it before I got a refund. And then I had to re-buy the green dress. I said 'yeah so you can send another purple dress!' 
Also as I live in a rural village, it meant having to go four miles to my closest post office, (in the snow, and in lockdown,) AND I was expected to pay for the return postage! I was so fucked off. I gave them a negative feedback, (and I'm not sorry, coz they deserved it!)
I kept the purple dress because it was just too much hassle to return it. When I looked at the seller's feedback a few days after I left my negative, there were FIVE more negatives (after mine.) All said 'wrong colour item sent; seller refuses to acknowledge, and expects me to return item at my own expense if I want refund! AVOID.'
I do feel for you OP. YANBU for sure.