adafruit_imageload + PNG

hi!
I have a png file (source here https://developer.lametric.com/content/apps/icon_thumbs/52160)

`bit_depth=2`
```
import board
import displayio
import rgbmatrix
import framebufferio
import adafruit_imageload

displayio.release_displays()

matrix = rgbmatrix.RGBMatrix(
width=64, bit_depth=2,
rgb_pins=[board.GP2, board.GP3, board.GP4, board.GP5, board.GP8, board.GP9],
addr_pins=[board.GP10, board.GP16, board.GP18, board.GP20],
clock_pin=board.GP11, latch_pin=board.GP12, output_enable_pin=board.GP13,
doublebuffer=True)
display = framebufferio.FramebufferDisplay(matrix, auto_refresh=True,rotation=180)

display.rotation = 0

g = displayio.Group()
b, p = adafruit_imageload.load("images/w-rainy.png")
t = displayio.TileGrid(b, pixel_shader=p, x=20, y=10)
g.append(t)

display.root_group = g

while True:
pass

```


some changes....
`bit_depth=6`
```
import board
import displayio
import rgbmatrix
import framebufferio
import adafruit_imageload

displayio.release_displays()

matrix = rgbmatrix.RGBMatrix(
width=64, bit_depth=6,
rgb_pins=[board.GP2, board.GP3, board.GP4, board.GP5, board.GP8, board.GP9],
addr_pins=[board.GP10, board.GP16, board.GP18, board.GP20],
clock_pin=board.GP11, latch_pin=board.GP12, output_enable_pin=board.GP13,
doublebuffer=True)
display = framebufferio.FramebufferDisplay(matrix, auto_refresh=True,rotation=180)

display.rotation = 0

g = displayio.Group()
b, p = adafruit_imageload.load("images/w-rainy.png")
t = displayio.TileGrid(b, pixel_shader=p, x=20, y=10)
g.append(t)

display.root_group = g

while True:
pass
```


so. my question is - what I am doing wrong?