L colour is the eighth grade in the GIA diamond colour grading scale, falling within the Faint category alongside K and M. The GIA scale runs from D (colourless) through Z (light yellow or brown), and the K-L-M grades represent the boundary at which body colour becomes detectable to most observers under standard grading conditions, particularly in face-up viewing of stones over approximately one carat.

L-colour diamonds typically show a discernible warm tint, a faint pale yellow (or occasionally faint brown) body colour visible against a white grading tray when the stone is viewed table-down. In the face-up position, smaller L-colour stones (under half a carat) often appear nearly white to the casual observer, while larger stones show their colour more clearly. The visual difference between L and the next grade up (K) is subtle and detectable only under standard conditions; the difference between L and M is similarly subtle.

In trade pricing, L colour falls below the bridal-quality threshold for many North American buyers, who target H or above for engagement rings, but the grade is widely accepted in European, Asian and Latin American markets and offers strong value for clean, well-cut stones. Yellow gold settings can mask the warm body colour effectively, and many L-colour diamonds in 18-karat yellow gold settings appear visually equivalent to G or H stones in white-metal mounts.