Well, the Zenirix is apparently a pretty old pet. xD I've seen staff say that we only have a green dog for that reason, and that it's really not very like the Res style now.
And yeh. Neither hatching dye colors or getting twins half the time seem like very good ideas to me. Both are a danger to economy, actually. On one hand, you get free dye colors, which would NOT be good; on the other, you have twice the pets being hatched, on a regular basis. We already have problems with too many pets. We don't need to fuel the fire and pretty much give people extra hatches.
The novelty would wear off too. People wouldn't want to get twins anymore. The only reason most people love albinos is that they're rare. Twins may even become like naturals - shunned by the people for their commonness, making it a ha-ssle every time you wanna hatch. Oh, you only wanted one? Too bad.
And really, it's just not realistic at all, having it that common. Twins are rare enough in Humans, and they don't have to be stuck in an eggshell the whole incubation, trying to survive on a food source meant for one. Double-yolked eggs may not be entirely rare, but actually hatching them? Definitely not a common thing.