Opal, chinchilla and chestnut are all agouti pattern colors. The agouti pattern puts white on the insides of the ears, around the eyes, around the nostrils, under the jaw, on the belly, and on the underside of the tail. There will be a triangular patch of lighter colored hair behind the ears. The body hairs of an agouti patterned animal have distinct bands of color.
In this country, "agouti" is a pattern, but there is no recognized breed of rabbit that has "agouti" as a color. The closest you can come to that is a few breeds (like the lops) that have a color called chestnut agouti, which is basically the wild-type color. In a lot of other breeds, this color is simply called "chestnut."