Shine outlines are invisible outlines forming the shapes of Shine Sprites. They come in different colors and sizes, usually found in the sand and in walls. Mario may spray an outline to reveal it and unearth whatever is below it. Despite forming the shape of a Shine Sprite, fully revealing an outline, excluding the case of the Shine Sprite in the Sand mission, usually reveals a Blue Coin.
Said secret mission in Delfino Plaza is entirely centered around locating the area of a small outline on the beach, found shortly east of where the lighthouse stands. After spraying the outline, the image is also filled and colored in, sprouting an actual Shine Sprite from it. Once the Shine Sprite has been collected, from this point on, the image remains fully displayed on the shore.
Four large, tan outlines are spread out along Gelato Beach. All of them can be found in any mission, excluding one in the first mission. These outlines are found near the Sand Cabana, between the peninsula and Sand Cabana, near the Surf Cabana, and in the center of the peninsula, respectively. Fully revealing each of these outlines shoots out a Blue Coin for Mario to collect.
There is another hidden blue outline on the wall next to the tower in Ricco Harbor - filling it in gives Mario another Blue Coin.
A final blue outline may be found in one of the bedrooms of Hotel Delfino, in certain episodes. Mario must sneak in and reveal the outline in the blank portrait to receive a final Blue Coin.