How is colour graded in colourless lab-grown diamonds?

Colourless lab-grown diamonds are graded on a letter scale from D to Z, where D represents the absence of visible colour and Z indicates a clearly yellow or brown tint. IGI, the primary certification body for lab-grown diamonds, applies the same scale used for mined stones, so grades are directly comparable. The scale groups into six descriptive tiers: D and E are finest white, F and G fine white, H white, I and J light tinted white, K and L tinted white, and M through Z tinted colour.