Octomap /projected_map color is inverted

[SnoopyGeeek]

I use octomap server to build 3D map and 2D map in the turtlebot3 simulation environment. After hang around in the house environment, I found the the color of the map that I have constructed is inverted. Usually light color means traversable, deep color means obstacle, mine is inverted.

Here is my map in rviz:

Here is the map I have stored using map_saver (pgm file):

Note: I did not use negate option.
Can anyone plz help me out?