how custom discreate colormap work with expression

[satas12825]

I'm attempting to create a custom colormap for my NDVI raster, but I think I might have misunderstood something, as the results aren't quite what I expected. Could you please help me figure out what might be going wrong?

first i try
ndvi_cmap = {
-1: (0, 0, 255, 255), # Blue for non-vegetation
-0.5: (0, 255, 255, 255), # Cyan for barren land
0: (255, 255, 0, 255), # Yellow for sparse vegetation
0.2: (200, 255, 0, 255), # Light green for low vegetation
0.4: (100, 255, 0, 255), # Green for moderate vegetation
0.6: (0, 255, 0, 255), # Dark green for dense vegetation
1: (0, 128, 0, 255) # Very dark green for very dense vegetation
}

then try to rescale value to 0-255
ndvi_cmap = {
0: (0, 0, 255, 255), # Blue for non-vegetation (e.g., water)
63: (0, 255, 255, 255), # Cyan for barren land or built-up areas
127: (255, 255, 0, 255), # Yellow for sparse vegetation
153: (200, 255, 0, 255), # Light green for low vegetation
178: (100, 255, 0, 255), # Green for moderate vegetation
204: (0, 255, 0, 255), # Dark green for dense vegetation
255: (0, 128, 0, 255) # Very dark green for very dense vegetation
}

and then use colormap like this
expression="(b3-b1)/(b1+b3)"
colormap = cmap
# Register the custom NDVI colormap
colormap = colormap.register({"ndvi_cmap": ndvi_cmap})

        cm = colormap.get('ndvi_cmap')
        with Reader(url) as cog:
            img = cog.tile(x, y, z,tilesize=512,expression=expression)
            # img.rescale(in_range=((-1,1,))
            img.rescale(in_range=((-1, 1),),)
        content = img.render(colormap=cm,img_format="PNG")
        return content