Organic natural colors with floral and earthy patterns.
Think linen, chiffon, pastels, and soft tones.
Please keep in mind the ceremony and reception will be mostly on grass.
Some Inspiration:
https://www.pinterest.com/ashleycamilleb/garden-party-semi-formal/
https://nz.pinterest.com/emma_joss/garden-party-semi-formal-attire/
https://ca.pinterest.com/nadinesnpdr/semi-formal-garden-party-attire/