Answer: Sedum Autumn Joy takes its direction from nature and dies when a heavy frost occurs. Some years this is earlier in the season than others so some years the plants will have enough time to develop deep flower color and other years they won't. There's nothing wrong with your plants, they simply did not have enough time to develop the rich dark color you expected. Next year may be different.
A 6 by 2' bed should provide adequate room for the plant combination you have. When things get too crowded you can dig and divide the plants, amend the soil, then replant.
Best wishes with your garden!
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