There are five developers hiding throughout the world.
Each one you kill will unlock a new color palette that can be accessed from the options screen.
You have to kill all five in order to unlock Ending 87.
Finding the 5 Developers
Climb up the West Cliff and head all the way to the left until you get the to Shop. Jump out the back window. and climb up the platforms of the Quiet Hill. The left side will have a hidden cave with this developer, as shown in the image above.
This will unlock the “Pala8” palette.
At the top of Macro Mountain (the mountain to the right of the castle,) you will find this developer in a hidden cave shown in the image above.
This will unlock the “Green Boy” palette.
After rescuing the princess, fall in the new opening below where the swinging anvil was. Hug the right side and go through the door here for this developer.
This will unlock the “Upwell” palette.
Collect the Chicken and head to the Mountain Base to the right of the castle. Pick up the Bombs and blow up the rock to the right. Enter this cave and cut away at the leaves on the right side of the Mossy Cave. Jump into the wall on the right for a hidden passage to the Great Waterfall. Flip the switch in the lower left corner of the Great Waterfall and cross the bridge that extends to the right for a hidden passage. If you fall, you’re better off restarting. Head to the dark bridge and fall off. Immediately move to the left and land on one of the platforms there. Jump across these platforms to the left until you reach the dev in a hidden room.
This will unlock the “Spectre” palette.
You should complete Ending 60 before going for this developer for the sake of convenience. Collect the Sword of Legend and Hookclaw. Enter the castle and fall through the hole in the floor to the sewer. Head to the left for the Abandoned Bridge. Press the button to extend the bridge. You want to fall under it and keep using your Hookclaw to reach the right wall. There’s a hidden area under the bridge to the right. Follow this path to the end to enter the Music Room, where you’ll find this developer.
This will unlock the “Outline” palette.
Getting Ending 87
After killing all 5 Developers, visit the graveyard to the left of the waterfall.
You don’t have to kill the developers in one life, you can kill them in any order and at any point as you progress through the game. After all 5 are dead, head to the right to enter the Mountain Base. Pick up the Bombs and blow up the rock to the right. Climb into the Great Waterfall and fall down. Swim to the left to enter the Waterfalls. Head down the platforms and you’ll see an open door to the left. Enter it and fall down the pit in this room.