Most gardeners know beans germinate best in warm soils, but did you know that certain types of beans germinate better in cool soil? According to John Gale at the Stokes Seeds, Inc., dark-colored, brown, black, or purple snap or pole beans such as 'Tendergreen' and 'Top Crop' absorb moisture more slowly than other types, and they can survive longer before germinating in cool soils than white-seeded beans like 'Derby' or 'Contender'. The white-seeded varieties take up water too quickly, causing the seed to split and rot in the cool soil.
The moral of this story -- For the season's first bean crop, plant dark-seeded varieties. Wait for warmer weather to sow seeds of light-colored beans.