One number is exact; the other is not, and this page is honest about which
Karat is a legal and arithmetical fact. 18 karat gold is 75% pure gold — 18 parts in 24 — by definition, and that figure does not move regardless of who made the piece, what colour it is, or where it was sold. This site's karat converter and every melt-value calculator on it rely on exactly that fixed relationship, and it is why they can quote an exact figure rather than a range.
What is not fixed, anywhere, is the recipe for the remaining fraction — the 25% left over at 18K, or the larger remainder at lower karats. That portion is split between copper, silver, zinc, nickel and palladium in proportions each manufacturer sets for themselves, to hit a target colour, hardness and casting behaviour. There is no law or trade body publishing a standard rose-gold or white-gold recipe the way the UK Hallmarking Act publishes 750 for 18 karat. Search for one and you will find plenty of confident-looking percentage tables online — and they disagree with each other, sometimes sharply, because none of them is quoting an actual regulated figure. This page gives the mechanism instead: which metal pulls the colour which way, and why, without pretending to a precision the industry itself does not publish.
The mechanism, metal by metal
Pure gold is yellow. Alloying it with copper pulls the colour toward red — more copper, deeper the red, which is the entire difference between a subtle "rose" gold and a much redder "red" gold sold by some makers; there is no fixed percentage boundary between the names. Silver pulls the other way, toward a paler, slightly green-tinged yellow, and is used both to keep plain yellow gold from skewing orange and, alongside copper, to soften rose gold's red toward pink.
White gold works differently: nickel or palladium is added specifically to cancel gold's natural yellow rather than to shift it toward another hue. Even a well-alloyed white gold comes out of the crucible a pale greyish yellow rather than true white, which is why almost all white gold jewellery — regardless of maker — is finished with a thin rhodium plating for the bright white most people picture. That plating wears thin over years of contact, and a white gold ring quietly warming toward yellow is the plating going, not the gold underneath changing.
Green gold is the least common of the four, made with silver and zinc and little or no copper. It is a subtle effect — closer to a cool, slightly muted yellow than anything resembling grass-green — which is part of why it stays a minority choice.