The documentation could state how many elements need to be in the vectors. Ideally, every component could have any number of elements, and if there are too few, the other elements are interpolated.

But it is probably simpler to just state "The number of elements in each vector must be 256 or 1024, depending on the depth of the framebuffer (24 bits or 30 bits)."

X11 uses 16 bit values even for 24 bit screens, because the actual values that are sent to the DAC can have higher precision than the screen pixmap. I doubt that Wayland has reduced the precision, so I don't think checking values here is right.

Maybe static_cast?

I haven't used wl_array but can't you allocate big enough contiguous chunk of memory and call memcpy, e.g.

wl_array wlRed;

wl_array_init(&wlRed);
auto* redDest = wl_array_add(&wlRed, sizeof(uint16_t) * red.count());
memcpy(redDest, red.data(), sizeof(uint16_t) * red.count());

Good point about allocating directly all of the memory. I looked up the wl_array definition and should be no problem.

Number of elements depends on the OutputDevice technical possibilities (i.e. for example as you said 256 for 8bit color ramps and 1024 for 10bit ones and every other value between 1 and 16bit ramp size).

Will add a comment to the doc.

I forget to remove this comment. This is not an issue since we can "stretch" the image over the whole uint16_t value domain linearly such that image 1 is associated with the largest uint16_t value. By that we have well defined arguments in any case.

The array size though is checked before that in the checkArg call by comparing it with the size set first by the compositor.

You can do this all in once

destination->resize(curve->size);
memcpy(...)

You forgot const. :-D
Also, I would use size_t instead of int.

That's not C so you can get rid of struct keyword. ;-)
Also, you could constify curve to show that it's read-only.

constify newColor?
Also, you could cast oldColor.size() in C++ manner, e.g. static_cast<size_t>(...)?

constify wl_array?
constify pos, e.g. uint16_t const *pos?

constify origin?

Can't you memcpy here?

You can resize v and call memcpy (or use std::copy?)

nitpick: added unrelated new line

you need to do a version check and only send to clients which have the required version. A FOO_SINCE macro should be defined by the generated wayland header.