"Blue Core, no, Indigo Core Stage? That's quite surprising…"
The color of the rainbow, or rather, of visible light as its wavelength gets smaller and smaller, Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Of course, there are several other colors that one can see through the prism, yet these seven are the ones that stand out the most, and these seven, along with the Silver Core and Golden Core Stages, comprise the progression of the Western Continent's mana cultivation system.
Going at it by the order of steps, one can say that what Liliana is seeing, the Indigo Core Stage, is something that anyone in the continent will find impressive, impressive even more if the one with such high level of cultivation are people just of the same generation, of the same age as her, branding them as geniuses who will define the next era of the continent.
