Castor is the dominate color but is very difficult. There is a lot of variation in the color even with "correct" color. There are lots of factors that go into the mix to make a good castor color like rufus modifiers, band width, and amount of tipping. Personally I like a heavier tipping with a wide, high rufus mid-band. That combo produces the perfect Castor color for me.

Judon's "Fred" Thank you Don & Judy Atchison. A_ B_ C_ D_ enen
AMS Rusty Thank you Amanda Shelton. A_ B_ C_ Dd enen	Judon's "Ginger" Thank you Don & Judy Atchison. A_ B_ C_ D_ enen

Following is a description of the Castor variety as set forth in the ARBA Standard of Perfection for the Rex breed.

COLOR: 10 Points

Castor:

Color is to be rich, dark chestnut or mahogany brown, as even as possible over body, head, and legs. Intermediate ring color to be a rich orange or rufus red, clearly defined over slate blue undercolor. Fur to be lightly and evenly tipped with black. Belly color to be white or tan over blue undercolor. Eyes - Brown.

Faults: Heavy black top; gray effect instead of brown; white tipped, giving salt and pepper effect; barred front feet; failure to show blue undercolor on belly.

Standard of Perfection. Bloomington, IL: American Rabbit Breeders Association, Inc., 2006. 185.