Obviously clear or translucent, with no color to speak of is the best color for transmitting light. When growing vegetables, you want all the light possible. Greenhouses heat up rapidly, even in cold weather. A sunny day in February can send a greenhouse into the 80's or 90's F in less than an hour. If everything is frozen outside, and you want to keep your trees dormant, this is a bad situation. So a greenhouse for growing vegetables, is exactly the wrong place to winter tree. Plywood is a good color for a wintering structure for dormant trees that you want to keep dormant. Heating from sun is minimal. This is actually what you want. You do not want your trees breaking dormancy before the trees in the landscape break dormancy.
Dark green translucent roofed storage sheds are okay for wintering trees. Not much an advantage over plywood, except you won't need a flashlight during daylight hours.
If you want your trees to start growing early, use the clear or translucent glazing as you would for vegetables. Install fans adequate to move the entire volume of air of the greenhouse every 1 to 3 minutes, have it attached to automatic vents so that you can exhaust excess heat on those bright sunny January and February days. Heat accumulation is the number one killer for greenhouse growers. A battery or generator system to open vents during power outages is also essential for a clear glass or translucent greenhouse.
Personally, I've had the least trouble using a plywood shed to keep my trees dark and frozen until it is time to put them outdoors.
