If the seeds are stored in a cool, dry place, they are likely still viable for planting that year. The seeds have a hard coat, and it's a good idea to file or nick the coat with a knife to encourage germination. Seeds are planted outdoors after the soil has warmed, so you can sow them now. Or you can plant the seeds in individual pots that will accommodate a long root. Either way, the soil should be well-draining.
Some gardeners drench the soil with a fungicide before sowing.
Just last week I began to see numerous sulphurs in our garden. The orange sulphurs display black borders when the wings are spread but are greenish-yellow on the undersides. The orange sulphur likes clover and vetches. The caterpillars, the Tvetens write, are a bright green and have a white line along each side. The caterpillars of the yellow cloudless sulphur are not very distinctive.