In watchmaking, red gold is not an accessory aesthetic choice: it is a precise metallurgical decision, with direct consequences on hardness, behavior over time, and workability of the alloy. The color derives from copper, and the amount of copper in the alloy determines both the intensity of the red and the mechanical properties of the finished metal.
The base is always 18 carats — 750 thousandths of pure gold — but the designation changes depending on the purity and composition of the remaining quota. The marking 5N appears often in technical documents from IWC, H. Moser & Cie and others: in Switzerland it indicates 18K red gold with purity declared at five nines (99.999%), with a copper percentage that pushes the tone toward a warm and pronounced red, clearly distinct from conventional rose gold. Moser uses 5N in the Streamliner and other models where the tint must stand out visually from steel without resorting to surface treatments. IWC employs it in the case, bracelet, and decorative elements of some Ingenieur and Le Petit Prince models. The marking 6N appears instead in contexts where the copper component is further calibrated to accentuate the red; the practical distinction between 5N and 6N is real but subtle, and in the laboratory it reads more to the touch — in surface hardness — than to the naked eye.
Omega developed a proprietary alloy called Sedna, registered in 2012: 18K red gold with palladium added to stabilize the color over time, slowing the surface oxidation that tends to shift rose gold toward yellow over the years. I don't know the exact complete formula — Omega doesn't publish it — but the stated objective is to maintain color saturation longer compared to traditional alloys with only gold and copper.
From the bench I've seen many red gold cases return for service after ten, fifteen years: the difference between a well-balanced alloy and one with too much copper is read on the corners of the case, where the softer metal yields first. For this reason serious manufacturers calibrate copper with precision and add traces of zinc or palladium to compensate for the loss of hardness. Red gold is softer than steel but stiffer than pure yellow gold: it machines well, responds well to mirror polishing, but must be handled with caution on sharp edges.
The red triangle at 12 o'clock on Flieger models and the red rotor on Oris have nothing to do with this alloy: they are applications of red color as a visual signal, a completely different category. It's worth keeping them distinct.