All I do is this:
step 1: small canvas on ibispaint
step 2: import sprites
step 3: each layer = each color
step 4: save each layer as transparent image
step 5: import ever image layer into castle
step 6: make sure to choose 2 color limitation for each layer because that is the lowest and we don’t want 5 colors on one layer
step 7 (optional): once you finish, merge them all down.
either that or I use dither