So by default, when you install a UCJV300 it sets up with "Mimaki Expand Color" as the preset in the Color Matching section of Rasterlink 6 by default. While this is good for punchy vector colors, it's not great for reproducing skin-tones or other more subtle photo images. Create a new Color Matching Preset and then select "Perceptual" as the rendering intent for both Vector and Raster images, and then choose "MimakiWideCMYK" and "sRGB" as your input profiles (you could also use the Profile manager to load "SWOP" or "GRACoL" input profiles too, to match your workflow, but the MimakiWide and sRGB work well for most users.
Just so you know, the "Mimaki Expand Color" preset that sets up by default uses "Relative Colormetric" as the rending intent (which has no black compensation and causes your shadows to block up) and for the input profiles, uses "MimakiHighContrast" as the CMYK input which pushes all of your colors towards the outside of the gamut (i.e. max saturation), creating colors that are a bit "clownish"
If you're looking for custom color profiles, this is a service we offer remotely (as well as onsite). Providing we have the same ink set in one of our demo printers we can produce a profile on the media of your choice, and targeting your specific needs (i.e. neural greys, spot color matching, etc.)
Hope this helps!