vocal colors presets do not mach previews

All these patches are very dynamic and depend a lot on how they are played. Since a preview is a static recording made using a short standardised MIDI file printed to wav, then converted to ogg, it can't sound exactly as played for patches like this, although you can get pretty close if you adjust the volume to match the preview volume and play without much expression. Previews just give you an idea of what the sound is like but they are not a substitute for auditioning sounds.

Omnisphere is a completely different case - it is not a plugin host so the preview it is giving you is not a static recording, it is playing back the actual patch. For KK to give you actual playback type previews it would have to load the plugin for each patch, which in some cases would take too long and you might as well just be auditioning them manually anyway. I think your expectations are unrealistic - in most cases previews sound close enough to give people a reasonable idea of what to expect, for the library you mention where the sound is very dependent on how the patch is played it is always going to be less close than something simpler like a Monark or drum patch. Every preview is made with the same standardised MIDI file, same velocity levels etc (except for things like drums or generative synths where the MIDI file needs more of a pattern), so those that are very dependent on things like aftertouch and expression (which these do) may well sound more different to instruments that work better with fixed velocity and no aftertouch.