Which color cannot be produced in Roses, even through genetic alteration?

The story behind the answer

Blue. Roses naturally make red/orange anthocyanins (pelargonidin, cyanidin) but lack the enzyme F3'5'H needed to produce delphinidin, the pigment behind true blue. Even when scientists inserted F3'5'H from other plants (e.g., in Suntory’s “blue” rose), the petals turned lavender, not blue, because petal cell pH, co‑pigments, and metal ion binding—factors essential for stabilizing a blue hue—weren’t right in roses.

By contrast, “black” roses are really very dark red, and orange or brown roses are achievable by mixing anthocyanins with carotenoids through breeding. Memory tip: Roses can do R and Y but not true B—think “RY” from the RYB paint model. Without the biochemical toolbox and petal environment for delphinidin-based coloration, a genuine blue rose remains out of reach.