[cairo] Subtractive API, part 1

[ecir hana]

(This is just a thought experiment, it's far from complete, I plan to
make it more complete in time - especially tints and multiple spots.
But I thought we could try to tackle the problem from another side, to
strip it to bare minimum and see where it grows.)

Suppose you want to define a solid (100%) golden metallic spot color.
Just one spot color and no shades. If I proposed:

void cairo_set_source_spot(cairo_t *cr, const char *name);

what do you think it is missing?

Obviously a fallback in a case the color in not available on the
device. What else? Spot colors don't have ICC profiles and you cannot
display golden color accurately, anyway. Therefore, what about:

void cairo_set_source_spot(cairo_t *cr, const char *name, double cyan,
double magenta, double yellow, double black);

I know this is somewhat similar to the previous discussion but,
please, could you tell me what do you think is missing from this
proposal?