The fifth of the nijuushi sekki is Seimei, "Clear and bright". Seimei occurs when the sun is exactly at 15 degrees celestial longitude, and ends when the sun reaches 30 degrees. On this day, skies are usually very bright and clear, according to tradition.
Since it is said to be so bright, why not wear warm colours, such as bright yellow-greens (think new shoots), sakura-iro, maybe a little red or orange to highlight the nice day? Think butterflies, warm breezes after a long winter, falling flowers, and fluttering sounds of windchimes. Think of beautifully blue, cloudless skies, and chirping birds.
As for me, I would probably wear a brown kimono with cream, gold, and pink accessories. Maybe camellia patterns, and a small but flashy jewelled brooch for an obidome. What would you wear?