It really depends on what colour and fabric you have in mind. You could go for orange but it's a bit obvious. Is it for winter or summer? Do you like wool or would you prefer cotton, linen or a blend? A deep green might work or a stony neutral. Budget wise there is so much choice it depends on how often you wish to replace it and whether quality is important. Ikea has some nice ones at the moment (seasonal) as does H&M and Marks and Spencer. I have summer and winter throws.

https://www.etsy.com/uk/market/sofa_throws

I’ve got a lovely wool throw from Nordic Nest. They do tons of beautiful wool or cotton throws in lots of colours. Prices about £30 - £50. Found this orange one, but if you prefer a more neutral shade they have lots.

Consider a colour that complements the existing decor. A warm, textured throw in a shade like burnt orange or rust would coordinate with your rug and add depth to the neutral tones of your sofa but like Isla says, it may be a bit obvious. Alternatively, a soft, neutral throw in grey or beige can provide a cosy contrast while maintaining a sophisticated look. John Lewis or The White Company offer a variety of high-quality throws that would suit your space perfectly