Hi Charlotte, I am not sure what your taste is like but you actually have quite floral colours with the green and the purple I have added some photos of bold florals to tie it all together this adds a shot of colour and coming into summer will brighten up any room.

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Abstract Floral Designer Plain, Printed & Floor Cushions | bluebellgray · More Info

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Don't mean to be cheeky.....is your sofa purple or grey? looks purple in the pics

Greyish

You can choose anything you like. It would be good for us to see more of the room, so we can see what style will suit.

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I think colourful cushions would be nice as in Naomi’s examples. I think a total of four, say two either end hugging each other would look nice. Maybe two plain and two patterned?

Its stil

You could pick some colours from the artwork? I'd probably go for shades of pink and maybe some orange (but muted). Here are some examples from LaRedoute and Next

Ooooo I love that!!!

Check out LaRedoute. They have some lovely things.

I've got a few pinterest and houzz boards with home furnishings on them that may be of interest: https://www.pinterest.co.uk/PinterestByLee/great-home-textiles/