What is this background colour in on-site collections?

[Purrder]

Pardon my ignorance, still semi-new to all things pet battles. But when looking through my collection I noticed some pets have a normal white background and some have an orange-ish background/overlay thing, can someone tell me what that signifies?

[Melmo]

The pets with an orange background signify a duplicate pet that doesn't count towards your pet score.