intense, vivid color is
(a) not usually associated with cmyk. Some colors are flat impossible to
get with cmyk. Such as the pretty "computer blue"
(b) when the subject image allows such, it will work only when color
corrected by someone who knows exactly what they are doing. I doubt
there are many on this forum that do.
(c) Hexachrome reproduction (special 6-color reproduction using special
inks) is a possibility, but is a special procedure/process and I don't
think MS Publisher is capable of handling it (nor can most other
applicaitons)
(d) Personaly I have never liked Publisher's CMYK output, preferring
instead to let it kick out rgb and then using my rip to convert. This
way I can control black generation.