What you call primaries (RGB, CMYK, XYZ) are in fact color models. Let’s just talk about the RGB color model for now as it is the most widely used in CG/VFX and the one used by ACES. The RGB color model is an additive one. The wikipedia page says this:
The full gamut of color available in any additive color system is defined by all the possible combinations of all the possible luminosities of each primary color in that system. In chromaticity space, the gamut is a plane convex polygon with corners at the primaries. For three primaries, it is a triangle.
So the primaries are the coordinates of the primary colors defining the gamut (the triangle) of possible colors. In VFX jargon (color scientists will disagree with this) we need three things to define a colorspace:
- A transfer curve (also called transfer function, OETF/EOTF or just curve). This can be a gamma curve, a log curve or a linear curve.
- A gamut, defined by the primaries. Is the maximum green of that colorspace a very saturated green, or a less saturated one?
- An illuminant (also called white point). If you turn on all three primary colors to full intensity, you will get a white pixel. But will that be a warm white or a cool white?
For the sRGB colorspace, the sRGB curve is (close to) a gamma 2.2, the sRGB gamut is pretty small with not very saturated primaries, and the illuminant is D65 which is the color of average midday light in Western Europe / Northern Europe.
For ACEScg, the curve is linear, the AP1 gamut is very large and the illuminant is D60, which is a bit warmer than D65.
If you take an sRGB image and you linearize it, you end up with a linear image that still has an sRGB gamut and whitepoint.
In Nuke, the colorspace node has controls setup in three columns: the three components of a colorspace (curve, illuminant, primaries). But in the curve knob, you have access to entirely different color models, like HSV, CIE Yxy. This can be misleading. WHen you choose one, you can see that the illuminant control or the primaries control can become grayed out, because it makes no sense for that color model.