it looks purty!

Why did you paint it white instead of black? Wouldn't black assist with heat retention?

Na Yan --- The real answer is that Mark thought it would look nice in white, and I figured it was his project. The longer answer is that I've read differing opinions on the best color for a cold frame/greenhouse. White will reflect more light/heat onto the plants and ground, which can be helpful. Black soaks up heat, but wood isn't a very good thermal mass, so I'm not sure you'd get much long-term effect there. I suspect it's six of one and half a dozen of the other. If the frame were made of stone instead of wood, then painting it black would probably make more sense.

You could put place a layer of old bricks on the ground inside the frame and paint their top sides black. That would give you extra thermal mass and a surface that absorbs heat.

I'm undecided about whether one should put e.g. a layer of insulating foam under those bricks. It would help the bricks warm up faster, but it would also decouple their thermal mass from the earth.