creating a cmyk based colorline

[ernie-f]

Good addition, Gary.
That's a reason why many companies has there logo/house colors in a spotcolor like pantone or here in Germany HKS.
Many of these colors are out of a printable CMYK gamut.

[handrawn]

ok I will resist the temptation to go into additive and subtractive colour theory....

in painting with real paint, basically, adding white to a colour is a tint, adding black to a colour is a shade

digitally you manipulate saturation and value, and so I could point out it is an HSV colour model you should be using not RGB, let alone CMYK....

but xara allows you to do this with CMYK and that is what the OP asked...

I agree it makes little sense to do it in CMYK, but if i were working with CMYK I would be using affinity not xara [and the same goes for true greyscale colour model]