The cold triggers it, but the more sun the brighter the color. There are exceptions 'Tom Thumb' Picea orientalis needs shade, i.e. to keep the gold color. I'd say your Zundert will color brighter in years ahead. People talk about off and on years with their cultivars with some frequency.

Dax

A old GW post speculating what causes this color change.

Dave

Here is a link that might be useful: What triggers winter yellows

Most experts ( not me ) agree the first and primary cue for trees to change color and drop leaves is the days getting shorter, followed by the temperature dropping.

Intensity of color is often intensity of sunlight.

mark

i just got done cutting scion off of my gold coin ...

those on the north side.. more buried in the tree .... out of direct sun ..... were significantly less yellow than those on the sun side ...

in fact ... they looked chlorotic green rather than gold ...

it really surprised me ....

one would think.. all other variables were exactly the same on a given tree ... dont you think ...

i have 4 other plants that have not colored properly for years.. and i am still mulling your magnesium theory ....

ken

Most of my golden variegated conifers (Thuja plicata 'Sunshine', Chamaecyparis lawsoniana 'Golden Showers' for example) are way more golden on their south side than their north. The Thuja is barely golden on the north side and screamingly gold on the south. I have to remember that when siting these kinds of plants as I don't want to put them where the south side would be difficult to see. Just another gardening challlening...