I definitely think blue flowers it it has a buttery cream tinge. I would personally stay away from pinks (which can clash; you need a cool colour with butter).
Blue looks wonderful with both cream and yellow, and you have your "something blue!" Forget me nots are nice in a bridal bouquet, but might be a bit late in the season now ... But I think diminutive flowers would suit that more low-key style of dress - which, by the way, I love!
White will show up the depth of the colour so I'd mix cream miniature roses etc. Mixing in a few yellow flowers might make the dress look less yellow by comparison - though yellow it isn't a traditional flower colour for weddings as I think the meaning is jealousy!
I'd go for a summery, cottagey blue and cream bouquet of smallish flowers. i agree cornflowers are a lovely summer vibe.