At my SiL’s funeral we were asked to wear anything pastel, because she loved pastel colours, so although my trousers were black, my top was a mixture of pastel colours and the coat I wore was a camel colour. This was a winter time funeral, but you can adapt to the seasons by adjusting the weight of the materials you wear and whether a coat or a light weight jacket /cardigan would be more appropriate. (I must stress that I did not go out and buy anything extra - I had these items in my wardrobe).
I would suggest, in the circumstances you describe, wearing something muted but what you would be comfortable wearing - I’ve no doubt that you probably have something in your wardrobe already, without you having to go to the expense of buying new.
It’s looking as if next week is going to be warm, so choose something cool, but neutral, and remember, as the family has said, it’s remembering a life lived rather than a life passed, so if you knew him and knew what he liked, go with that.
I don’t know your colouring, but for tops, you could think about lilac shades, pale pink shades, creams, a combination of them? A light weight scarf can also add a splash of neutral colour.
Linen, if you have it, is a cool choice (but can crease, so a linen mix might be better)..
Condolences to you, and I hope you find the posts here helpful 💐.