Interesting. So I have been watching the fruity one for 3 years now, and it seems to have that yellowish tinge every year. I just thought it was its natural color. It does seem to have a lighter colored leaf even in summer. Below are some pix I've taken at different times of year. (Yes, I'm obsessed with the stupid tree :-). We did have a dry late summer, maybe that contributed. But on the other hand - look at the foliage color difference between the two hollies even in summer: 2016-11-12: 2017-01-12: 2017-08-16: 2019-05-22

I. decidua may not be strictly dioecious but they are indigenous on my property and some, in fact most, do not produce berries. So I would ask the source if you buy one. I have had a difficult time finding a source. The Green Plant Market grows them from time to time and the owner, Jim Rodgers seems to be very knowlegeable about native trees. They do apparantly have a weeping form and a gold berried form, but are "temporarily out" of the typical red berried tree. Because it is generally quite small as trees go, they list it under shrubs. http://www.wegrowit.com/greenplant/GreenPlantMarket.html

It seems to me that kadasuki is looking for suggestions for tree that does not lose it's foliage, and a variegated one in particular. It could be a coniferous evergreen or a broadleaf evergreen - terms very common I believe in the horticultural scheme of things to describe whether or not a plant loses it foliage. On the coniferous side there are many that can be recommended as noted in this thread. On the broadleaf side for Zone 6 it becomes somewhat of a challenge. There is a variegated magnolia virginiana - the cultivar name escapes me - and possibly a few variegated ilex. Both will defoliate during extreme minimum zone 6 temps, but should regain growth the following spring. There are some viburnum that are evergreen in zone 6, but these for the most part tend to be large shrubs rather than trees. Sort of a tuffy to recommend a variegated broadleaf evergreen tree for zone 6.

Of course, Blue Muffins need to pollinated by another compatible viburnum, even though their flowers are complete(both male and female). Search for: viburnum pollination blue (no quotes)in the shrub forum. You will find which ones will work. Candy viburnums might work. I just don't know, but you will need something. Ilex verticillata should have acidic soil (but doesn't need it quite as much as blueberries). If you live in a lot where the ground has been reworked by bulldozers, you will certainly need to ammend it. Is "berry heavy" really "Heavy Berry" holly? If so, that's a different species of Ilex and won't pollinate or be pollinated by the others. Heavy Berry is a very old Ilex opaca cultivar, and one of the cold hardiest, but may not make it up here. It is evergreen and again, acidy soil.