The D locus determines whether a rabbit's base color is dense or diluted (more washed out in appearance).
Dense is the most common and responsible for colors like Black, Chestnut Agouti, and Black Otter. Most rabbits are dense, as it is dominant and more common than its lighter counterpart, the dilutes.
Dilute is responsible for colors like Blue, Opal, and Blue Otter. When dilute is present, the color of a rabbit (and the base) will be more washed out, lighter, and usually lacking in the level of saturation as the dense version (reds will have a little less tone, oranges will be a little grayer, etc). Oftentimes the eyes are also affected, becoming a blue-gray color (though not always).
Although it depends on the breed, darker shades of blue are often preferred in the US (in cases like the American and Havana), which result in dilutes being closer to their dense counterparts. In other breeds such as the Lilac and Beveren, dilutes may be much lighter. There is a vast range of lightness that can be selectively bred.
Does breeding two dilutes make the offspring more washed out?
- Because of the gene's natural inclination towards making colors pale and light, dilutes are sometimes faulted in breeds where brightness of color is preferred (even in the dilute colors), such as with Tans where deep red markings are preferred regardless of whether the rabbit is dense of dilute.
Although it is sometimes believed that breeding a dilute to another dilute will wash the markings out more (that it's better to breed dilute to dense), this is a myth that needs busting! The dilute gene doesn't can only be off or on, there's no range of onness or offness. Having two dilute parents (as opposed to one) doesn't make the dilute offspring any lighter than they normally would be.
The range of shades, tones, and brightness that we see are caused by other modifiers, which can be selectively bred either way.