They don't unless it's using a Hakeijo rune which is primarily only found in IC sewer or bought with telvar or through guild stores.

Same with provisioning writs. They're using Perfect Roe which is a rare ingredient.

Don't think I've ever seen a gold alchemy writ.

For durables, the purple/gold matches the temper used, but the trait used, and the number of traits required also affect the reward. Enchanting values usually go with the Aspect rune, but the Hakeijo and Indeko have a larger effect. Two levels of Potency, too.

For other consumables there is no single purple or gold ingredient driving things. Delicacies may use gold, purple, and green ingredients in one Provisioning recipe, so the writ matches one of them, but that doesn't tell you much. Alchemy is all green ingredients making purple writs, so I don't know what the logic is.