I took cuttings from my geraniums and grew them over winter in my conservatory. Everything was fine. I started putting them out for a few hours in the day's sun and they immediately turned brown. Some of the leaves had yellow around the outer rim of the leaf.
Geraniums Turned Yellow After Being Placed in Sun
Certified GKH Gardening Expert
When moving indoor plants outdoors you need to harden them off. It sounds like the sun scalded your geraniums. It's best to acclimate plants in shadier areas first then gradually introduce them to the sun.
For more information on hardening off houseplants, please visit the following link: