They're all rex furred from rex furred parents only.
https://thenaturetrail.com/rabbit-genetics/color-c-series-chinchilla-sable-himalayan-rew/
So, the short answer is there's no way to tell what the babies will come out like without a thorough color pedigree of both parents.
The full color gene that produces most colors like castor, black, red, etc. is a simple dominant gene on the C locus and is THE most dominant gene - only one copy is needed to express it.
The albino gene is a simple recessive, and is THE most recessive gene on the C locus. So if a rabbit carries one copy it looks like a normal rabbit, and if it has two copies it is Red Eye White. REW can sneak in a single unexpressed copy for generations and then pop up at random. Some people prefer it because it prevents chinchilla colors from cropping up in your "full color" rabbit lines and it's easy to work with.
So if your full color rabbit (the castor) has one copy of recessive white (C/c) you'll get about 50/50 split full color and white. (At random of course.)
If your castor is hiding a different C locus gene (C/???) like Himalayan it's a total wild card. There's several genes here it could be. But they will all have SOME color.
If your castor has only full color genes (C/C) they will produce only full color kits and the kits will all carry one REW gene and one full color (So all kits will be C/c)
Of course, it's not going to give a perfect split in any case because it's random, and REW can be hiding any host of colors and patterns, even broken, and you have no way of knowing how that will interact with the castor, but that should give you a broad idea of what to expect.
The only thing a REW can't be hiding is himilayan, chinchilla, sable, or other c locus colors.