Useful sprite draw function for PICO-8 (and maybe Lua in general)

outline_sprite.p8.lua

--
-- draws a sprite to the screen with an outline of the specified colour
--
function outline_sprite(n,col_outline,x,y,w,h,flip_x,flip_y)
  -- reset palette to black
  for c=1,15 do
    pal(c,col_outline)
  end
  -- draw outline
  for xx=-1,1 do
    for yy=-1,1 do
      spr(n,x+xx,y+yy,w,h,flip_x,flip_y)
    end
  end
  -- reset palette
  pal()
  -- draw final sprite
  spr(n,x,y,w,h,flip_x,flip_y)	
end

Two comments: first, the function above does one unnecessary draw operation (when xx==0 and yy==0 in the loop - that whole draw call will later be covered by the draw final sprite call at the end)

Second, if you are willing to let go of some corners, you can get a pretty decent-looking outline with only 4 draw operations:

function outline_sprite(n,col_outline,x,y,w,h,flip_x,flip_y)
  -- reset palette to col_outline
  for c=1,15 do
    pal(c,col_outline)
  end
  -- draw outline
  spr(n,x+1,y+yy,w,h,flip_x,flip_y)
  spr(n,x-1,y+yy,w,h,flip_x,flip_y)
  spr(n,x,y+1,w,h,flip_x,flip_y)
  spr(n,x,y-1,w,h,flip_x,flip_y)

  -- reset palette
  pal()
  -- draw final sprite
  spr(n,x,y,w,h,flip_x,flip_y)	
end