I’m a Church of Scotland minister and white for a christening is definitely not a thing in my denomination - not for the adults at any rate. Babies often, but not always, wear a white christening robe. I’ve not seen it at the Catholic ceremonies I’ve attended either.
It is nice to get dressed up for the occasion, but not at all necessary. Wear whatever you feel comfortable and confident in.
My suggestion is that once you have decided on a dress, you then focus on the vows you will be taking and the meaning of the sacrament. It is always lovely to welcome families into the church, but it is even more lovely to see them become involved in church life and to get to know them and support them as the children grow.
It is very depressing for clergy and congregations to have families come to church all dressed up for a christening, then never to show their faces again. Apart from anything else, in my tradition, the congregation make a vow to support the parents and child. We mean what we say, but how can we fulfil our vows if we are just treated as bystanders in a stranger’s party and photo opp?
Please, see this not as a one-off event but as the beginning of something more precious and enduring than a dress.