K...good question. My feeling is: the container color (light or dark) has a lot to do with how much heat is absorbed.

When I have a question like yours, I eliminate all the variables by sticking a soil thermometer in the container and the ground and read what it says. Al

Could you germinate inside and then move outdoors? I've germinated lettuce and other greens indoors (72F) quite quickly. Just put container planted with seeds near a window and loosely cover with plastic wrap. Move outside (after hardening off) once seedlings have a few true leaves.

Containers can be incredibly hot depending upon sun exposure, container size, and how the container is sited (on risers versus sitting on a slab of blacktop). From what I recall, the color of a container doesn't offer as much of an advantage as I'd previously thought....a white container was only 5F degrees cooler than a black pot...if I'm remembering rightly.

Josh

Do the color test for heat on a car lot like I did some time. Hold your hand on different shades of cars...amazing!