preview shown in the browser is an RGB approximation to the CMYK color,
since all monitors are RGB and not CMYK. The color profile used to do
this conversion within our software is the USWebCoatedSWOP.icc from
Adobe. That is downloadable here:
However, please note that despite any differences in how the monitor may display the color, the production file is being defined with the exact CMYK value you set for the field.