What about a twill beige.
Or even a white twill slipcover that can be washed easily.
I like light and bright.

Navy is also coastal.

What about a nice white cowhide leather I have 3 huge dogs , entertain a lot have grandkids and mine are 14 yrs old still look like new BTW leather if it is good quality and on all sufaces breathes so 10 secs. to body temp in all seasons . The only thing do let let the kids near what ever you get with anything indelible

I'd use any shade of blue or a tan/taupe for the sofa, like the sand and sky. You can use your crisp white on the walls.

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Birch Lane has slipcovered couches which may be a good way to go. They offer different fabric choices, including Sumbrella. A light denim look could work well for coastal and I’m sure they have consultants who can provide durability advice and believe they provide samples.

https://www.birchlane.com/living/pdp/84-rolled-arm-slipcovered-sofa-with-reversible-cushions-b000748998.html

Occasional chairs could have a pattern (stripes for instance) for a casual look. Room below would look great with a blue slipcovered couch.

I think light gray can achieve a similar crisp clean effect but not be as susceptible to stains as white

Check out the Sunbrella line of interior/exterior fabrics. Many have a wonderful linen texture and they are indestructible. Others look like canvas. Their “Sand” color is a wonderful alternative to white for a coastal look. I have Sand colored canvas Sunbrella cushions on my sun porch furniture. And I have a terrier. It is working out very well!

there are so many performance fabrics out there that are pet and kid friendly if you really want a white couch i would go for it.

we have a white couch with 2 teens, 2 cats and 2 dogs and its held up well

most companies have these performance fabrics and list if they are pet/kid/stain friendly

How about white slipcovers in twill or a striped denim look? The slipcovers come off easily to be washed as much as you need.

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Washable slipcovers are the way to go, IMO.

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Sunbrella indoor/outdoor fabrics are incrediby practical, based on my experience.

I’d go with blue slipcovers.

@decoenthusiaste Have you been able to wash slipcovers and keep them the same size? Whenever I've washed them, even in cold water and line dry, they still shrunk.

I would go for a good quality white leather