The Zone VI modifications that Fred Picker, along with Dr. Horowitz, developed amounted to UV and IR filters, sharp cutting filters that allow the meter to read colors more accurately (as they are read by panchromatic film emulsions), and internal baffling to be resistant to adjacent bright light sources. I have used both the Pentax digital (unmodified) and the same meter later modified by Zone VI. My experience indicates that the modifications are beneficial in metering for black and white photography. I have no experience using the meter for color, since I do not do color photography any longer.
The matter of accurately metering green foliage is one of the IR emissions. If you will refer to an IR film photograph involving foliage, you will note that the foliage is very light in tonality. The reason is that foliage emits a large amount of IR. The unmodified meter will be sensitive to this IR and give a higher reading then what panchromatic film is capable of exposing (since it is not sensitive to IR). Therefore in using an unmodified meter when exposing foliage, the negative when developed will have lower densities. The print, from this negative, consequently does not well differentiate the tonalities in the foliage.