I have a few seedlings that are having some whitish leaves. What can be the cause of that? And what can I do about it? Or is it some kind of variegation of something like that?
Only 3 seedlings do have it.

Willeke,
Hard to see with the white perlite on top of soil, but it looks as though you could have a few seedlings w variegation. These often revert back to green as they get their true leaves. Hopefully it will remain for you.
As long as the leaves have healthy tissue and just the white coloration, then this is more than likely the case (variegation).
Rick

Rick, 2 of them is are getting a true leaf. That leaf is white with a bit of green in it. The other does have a true leaf, but that one sofar is totally white. The seedleafes do look healthy and green though.

I am sure it is variegation, Willeke. I had a few come out with variegation and sadly they reverted back. I hope they remain for you. Would be nice.
They say that they can revert back to green and then later on come back with variegation.
I marked my seedling just in case they do come back. Hope so.
Rick

I hope so for you Rick. Always nice to have some variegated ones. I hope they do stay this way for me. Just wait and see. I give them as much light as possible.

Willeke