The color was collected by the pigmentologist Chamelius Pendraggin, who found it in the other universe with the help of one of his Hue-Collecting Tomes, which he sent across diverse dimensions to steal colors from.
According to Chamelius, the color is new and unknown on earth. After he kidnapped Dipper in his book, he asks Mabel to help him. As a reward, he will free Dipper and show her the new and unknown color.
After Dipper and Mabel defeated Chamelius, he promises to show the color to the twins, if they promise to leave him and his book alone for good.
He also warns the twins that they mustn't stare for too long because human eyes weren't designed to take the color in. But it appears that the color isn't new or unknown for humans at all. It‘s blue with glitter. Dipper recognizes it because Mabel has a nail polish in the same exact color.
The color is blueish-grayish with glitter. The glitter contains little black, white, and light blue circles.
Although Chamelius claims it comes from a powerful, unknown dimension and is dangerous, Dipper and Mabel are not effected or impressed by it, because the color is the same as a nail-polish Mabel has and so is known on Earth.