The 57 high-opacity foundation colors the rest of a Citadel scheme is built on.

What Citadel Base is for
Base is the range you reach for first on a bare, primed model. These 57 colors are formulated for opacity: the whole point is that one or two coats cover the primer and give you a solid, even foundation to shade and highlight over. When a color exists in both the Base and Layer ranges under similar names, the Base version is the thicker, more covering one.
The spread here is deliberately broad rather than deep. You get roughly ten reds, a cluster of blues and greens, a run of eight near-blacks and dark tones, and the workhorse browns and greys that undercoat everything from leather to stone. This is not the range for subtle transitions; those live in Layer. Think of Base as the ten or fifteen colors you actually undercoat an army with, padded out with less-used foundations.
Because Base colors anchor a scheme, they are also the ones most worth converting accurately if you switch brands, since a highlight sitting on the wrong undercoat throws the whole model off.
Foundations worth knowing
- Abaddon Blackthe near-black most Citadel schemes undercoat from
- Corax Whitethe opaque white foundation for bright colors
- Macragge Bluea solid mid blue basecoat
- Mephiston Reda covering red base under brighter reds