The downside of using candy-coated eggs (or similar sweets) as a garnish is that they do 'bleed' their colour when in contact with cream. The shells will also go soft if refrigerated. I'm afraid it's just what this kind of sweet does. It's for this reason that I suggest adding the mini eggs right before serving. Every other aspect of this mini egg cheesecake can be made ahead of time, but just refrain from garnishing with the eggs until serving time. It's the only way to avoid this problem.