I made a design for a luffux in the generator and then used my essence for it, but I don't know what happened when it created it. I had black and a dark brown in as two of the colors, but it changed both to this weird dark blue on the actual pet.

What?

The pet: https://www.sylestia.com/view/pets/?petid=3595969

It seems like you used a regular essence. Basically, the essences that are worth less (regulars) when generated, have a chance of the colors not turning out the pay you set them to be. There is a slight color variation. This happens with lesser pet dyes too. (I've heard of someone who dyed black but it turned out white). If you use an enhanced or a mythical essence instead, the colors would generate much closer or exactly like the original. What happened to you is not something strange.

Anyways, I hope that helped! And if you have any more questions feel free to ask me.

Oh, I didn't know that was a possibility. Thanks for clearing that up!