Short answer
There are two Organ Improvers. The red-skull version needs runes totalling 4 red and 1 green (Iron Ore + Flax is one listed mix). The white-skull version needs 4 green (Flour + Flour). Both are brewed at Laboratory I after Alchemy, and they upgrade skin, bones, skull, heart, brain or guts — for example into Skin (Gold), Bones (Gold) or Skull (Gold).
The graveyard
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Rune totals
Red skull Organ Improver: 4 red + 1 green. Guides list Iron Ore + Flax as a working pair.
White skull Organ Improver: 4 green. Flour + Flour is the cheapest listed mix.
Any ingredients whose runes add up to the exact total work, so swap in whatever you have in stock.
Using it on bodies
Apply the improver to a body part at the autopsy line. It feeds six recipes, including the gold-tier Skin, Bones and Skull.
Better parts mean more red skulls, which means a stronger zombie. Pair the upgrade with a matching collar from the Jewelry Table.
Sources
Related pages
Official screenshot · © Lazy Bear Games / tinyBuildAutomationGraveyard Keeper 2 Alchemy Guide
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 PotionsOfficial screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Jewelry Table
Official screenshot · © Lazy Bear Games / tinyBuildProgressionGraveyard Keeper 2 Resurrection
Official screenshot · © Lazy Bear Games / tinyBuildProgressionGraveyard Keeper 2 Zombies