I'm unfamiliar with the castor color, but Opal is dilude of chestnut (aka Opal = blue agouti) and looking it up castor is a synonym to chestnut so yeah... my bad for not knowing that lol, I know it as chestnut is all.
What you'll get depends on the hidden genes of your rabbits. Agouti is a dominant trait that can hide self, so if both parents are carriers you, in theory, would get 75%agouti vs 25%self. If only 1 parent is carrier or if none are then you'll get 100%agouti.
Now chestnut/castor, is the non dilude and opal the dilude. If your buck carries the dilude this means you'll get 50% non dilude, 50%dilude. Although, this also means, if your castor doesnt carry dilude as a hidden gene, you will get 100% non dilude.
As for Broken, you'll get 50% broken 50% not broken.
Going further down the rabbit hole; Chestnut/castor = black agouti and Opal = blue agouti = dilude black agouti. Both being essentially of the black color means they could also carry chocolate wich is hidden behind black. If both are, then you could end up with 25%chocolate agouti and if both carry dilude too then a chance at Lynx. Lynx=lilac agouti. If both carry self too then even chances at chocolate and lilac. Also agouti can hide harlequin too etc etc etc XD
Your best bet is to just breed and see what you get, too many potential hidden genes without further information. But these are probabilities, you could end up with full chestnut/castor litters for ages making you think your male doesnt carry dilude and then one day get a blue out of nowhere. I've had that with one of my females, I've had her for 3 years and always though she wasnt a carrier for chocolate. Often bred her to a chocolate male and never got choco babies. This year, I breed her to a black male that carries chocolate and she gave me 2 chocos in the same litter shrugs. Sorry if this all just confused you further XD
