Alberto pointed out years ago that white plastic pots are better in hot climates, as soil and plant roots stay cooler. I garden in a cool summer location, so have stayed with darker colored pots for bulbs from warmer climates, with some success. In my cold frame however, I prefer white plastic pots for the plants I hold there in the summer, to keep the roots cooler. Clay pots have several drawbacks for me: they get very discolored on the outside, quite quickly, and take an annoying amount of time to scrub clean. They crack and fall apart in cold weather. Slugs and snails hang about on the shady sides, making forays up to nibble on plants in said pots. Kathleen PNW coast, zone 8, cool summers and mild wet winters