Almost anything. It's a really subtle grey-ish shade in lavender so it's really easy to match.
Yellow goes with blue nicely, will look very sunny and bright. Especially with a bit of white.
You could do acid green foliage and there's a really common grass I'm racking my brains to tell you [old age emoticon]
Pinks and purples will look just fine and very tasteful.
Even bright orange would be good.
Black grasses? with little pink flowers poking through? Very funky.
Bright red? That's the only thing I'm less sure about but it's such a subtle bluey grey in lavender no one would be like this: if you did.
Can't go wrong. Bit of ivy trailing over the side would be nice.
You could do lavender in the middle with smaller plants around the outside.
Or lavender with really dramatic big things at the back like foxgloves or gladioli. Depends on how much space you have.
And if you don't like it, you can dig them out and try something else next year.
World's your oyster. I'd just browse around the garden centre and get a few of whatever takes your fancy