Some varieties are yellow. I much prefer the pale lemon "Banana cream" to the white ones, but unfortunately the slugs and snails can remove every trace overnight...so they are no go here. If you want white ones you'll have to buy another plant and if you don't trust the label get one that's already in flower.

some white ones open pale yellow and fade to white as the flower develops.
have you had the plants long and if you had them in previous years is this just a change for this year?