I have a Pastel Meets and the special, light blue outline is actually gained by using the Easter egg the tamagotchi receives upon visiting Easter Land. This allows any character at any stage to not only have their outlines turned blue, but also take on a completely pastel colour scheme (and also reverse the change). The colour scheme differences are very distinct as the Easter egg can change green into mint and dark blue into light blue (those are the two I remember). It's a nice feature since if you wanted to easily change your tamas colour a bit, you just need to select the item. Unless a particular colour model has specific characters with non-black outlines, the option to change outlines probably doesn't exist.