The Black-Masked Lovebird (AKA Masked Lovebird or Yellow-Collared Lovebird) is the second most popular breed of lovebird kept in captivity (with peachfaces being the first most popular). They are around six inches tall with the typical lovebird short 'n stocky frame.
Since, like the peachface, they are a breeder favorite, they have a handful of color mutations available.
The average Black-Masked has a green body, thick yellow collar, and small black hood. They, like the Fischers', Nyasas, and Black-Cheeks, have the featherless eyering (meaning they can be successfully bred with any of the aforementioned species, but again, hybridization is frowned upon).
Like the peachfaces, they come in two base flavors: green series and blue series. Blue series (just called Blue-Masks, as people usually refer to the birds by body color rather than actual mask color) have the black mask, but their bodies are blue and their collars are white rather than yellow. Their beaks are also faded to a fleshy-beaky-pinkish-gray-ish color rather than red.
Their violet mutation has the black mask and white collar of a blue mutie, but with a rich blue-purple body color and gray rumps. Since violet (like the dark factor) can be inherited from both parents, the violet factor can be doubled, creating a really really purple! color. The color also gets stronger if there's a dark factor inherited as well, meaning that a violet can range from a deep blue-ish/purple-ish, to lavender to deep indigo. They also have the compatible pied mutation (random shades of the body color speckled all over. That picture if of a pied single violet-factor).
Like the peachface, they have dark factor mutations available as well (Mauve, slate, olive, cobalt). Mauve is particularly impressive as it results in a black face, gray neck, and dark gray body, ending up with an actual monochrome lovebird.
Lutino Black-Masked Lovebirds usually look an awful lot like lutino peachfaces (red face, yellow body), but the thing to remember is that the masked ones will have a red beak and the white eyering. Like the Fischer's, the Black-Masked Lovebird is capable of turning out an albino bird, and albinism is the most recently recognized mutation in the species.
Other mutations include: The dilute (yellow with pale orange mask), pied, and fallow.