Several possibilities come to mind. Did this discoloration occur right after you moved the plants outdoors? They may not have been hardened off, and could have gotten sunburn and/or windburn. It's a good idea to place new transplants in a protected location out of the sun for a week or so before putting them in their final location outdoors.
Another possibility is a touch of late frost. Zinnias are quite sensitive to cold temperatures, and if it the temperature got down into the upper 30s, they may have suffered from frost injury.