No such thing as an opal otter. Opals are an agouti color, otters are tan pattern. A rabbit can't be both. Both opal and blue otter have lighter markings on the nostrils, jowls, eye circles, triangle, belly and tail, but the difference is over the back and "dark" areas of the rabbit. A blue will be solid blue there, sometimes with a lighter gray undercolor. An opal will be blue over the top with some fawn pigment mixed in - they have a fawn intermediate band in the ring color. An opal is a diluted version of chestnut (castor), while a blue otter is a diluted version of black otter.
Here is a broken opal. You see a fawn intermediate band, with a blue tip to the fur and a blue base to the fur. (Photo from
Wildriver Rabbitry)
I couldn't find a good undercolor picture of a blue otter so I drew the difference: This is what it looks it if you part the hair over their back to see what the color is near the skin.
Color is definitely important at ARBA shows, but in some breeds it matters more than others. In every breed if the rabbit is not a recognized color, it will be disqualified from showing. In the vast majority of breeds, body type is more important than color. In some breeds like Mini Lop, color practically doesn't matter (as long as they are a recognized color - and in that breed, there are only a few unrecognized colors, like Harlequin or Otter). In some breeds like Netherland Dwarf, color does carry some importance but still not as much as body type. In only a few breeds, like Silvers and Standard Chinchillas does color matter more than type. There's a saying among rabbit breeds - you have to build the barn before you can paint it. It's best to make sure you have a line with good structure before you start breeding for color.
